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In D&D 5E, dim light only affects Wisdom (Perception) checks. If I'm interpreting the Player's Handbook correctly, for combat rolls,

  • creatures that don't have darkvision, whose target is in dim light, and
  • creatures that have darkvision, whose target is in dim light or in darkness (within range, so 60 feet for elves, for instance),

have no disadvantage on attack rolls.

This makes it difficult to use darkness and dim light in combat. You could argue that in a poorly lit dungeon, it would be more difficult to succeed an attack roll, especially in ranged attacks. But the ruleset does not provide a way to do it with a disadvantaged roll, as stated above, so I'm trying to find an alternative way to use light and darkness as a strategic asset.

Considering this, here is my question:
Would it be correct for the DM to interpret dim light as cover, if they wanted to use light and darkness as strategic asset for enemies when designing a particular map, such as a dungeon, ruins, tunnels, ...?
Or, if not strictly correct from a ruleset point of view, would it be acceptable as a "house-rule" (when taking care to warn the players about it, of course)?

If not, is there an alternative way to use light in combat strategy, considering a party may have many creatures with darkvision?

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    \$\begingroup\$ @Pat and tuskiomi: Please don't post answers (including partial or speculative ones) in the comments. Comments are for requesting clarification or for suggesting improvements to the question, not for answers. \$\endgroup\$ – V2Blast Dec 3 '20 at 22:36
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    \$\begingroup\$ @V2Blast That would have been useful nonetheless, now I can't see them anymore so ultimately it went against the very interest of this site. Over-zealous moderation is what is keeping me from answering on Stack now, or to ask questions when I can avoid it, and what may eventually kill the site. Just seeing how you had to change my question to have it the way you like made me want to drop it entirely, it really wasn't quite so necessary. Perhaps moderation/editing should be restrained to people who actually propose an answer, to avoid editing just for the sake of reputation score. \$\endgroup\$ – RedGlyph Dec 7 '20 at 17:12
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    \$\begingroup\$ Editing doesn't affect reputation, and my edit barely changed your question anyway... You're welcome to revert the edit if you like, though all it did was fix some formatting (and one typo). Regardless, though, if you have an issue with the moderation of the site (which is done by both the diamond moderators and the community as a whole), please take it up on Role-playing Games Meta. Note that you're unlikely to get much traction on the "answers should be allowed in comments" front; there are very good reasons why answers aren't allowed in comments, as explained in the Q&A on Meta that I've already linked. \$\endgroup\$ – V2Blast Dec 7 '20 at 18:54
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    \$\begingroup\$ ...If people want to leave answers, they're welcome to do so - in the answer field. \$\endgroup\$ – V2Blast Dec 7 '20 at 18:55
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It's not cover and will be awkward to interpret with a similar mechanical implementation

The official mechanics treat cover as a physical barrier to attacks-- a target behind cover has a smaller "hit box" than normal because the attack could hit the barrier instead of landing a meaningful blow on the target. This is represented with extra AC.

The tradeoff for that extra AC is (usually, in my experience at least) about tactical positioning: it's rare for a creature to enjoy cover and also be mobile. Exceptions of course exist, such as a fortified position designed to offer cover and mobility in a useful range.

I definitely understand the temptation to treat unfavorable lighting conditions similarly: a harder-to-see target is also harder to hit because it's harder to aim at them effectively. But in practice I have not been pleased with the results, which have primarily been that combats take longer without being more fun or interesting. When enemies have the extra AC and decent freedom of movement the situation is almost the same as just giving them a higher AC and not fiddling with the battlefield or enemy tactics at all. And a higher AC can matter a lot in how a fight plays out.


Poor lighting fits better with hiding and perception mechanics in 5e than it does cover mechanics.

The reasons I believe this to be the case are:

  1. There is no generic mechanic for precise aim (certain class features from Tasha's Cauldron of Everything not withstanding). The most specific target you can aim for, RAW, is a 5-foot cube. There are no called shots.
  2. Because the most precisely defined target you can try to hit is a square on the grid, knowing which square a target is in provides the maximum chance-to-hit scenario possible.
  3. When you don't know which square a target is in, there already exist mechanics for guessing their location on an attack roll in the Unseen Attackers and Targets section of the PHB (Chapter 9, Making an Attack, Unseen Attackers and Targets).

If you rule that it's harder to hit a target in poor lighting, applying Disadvantage represents that difficulty in a more streamlined way than designing a whole new suite of mechanics. This is exactly the kind of situation that Disadvantage was intended to address, and it's not more of a modification to overrule the bullet points noted in the question than it is to homebrew a bunch of new systems and rules.

The way that combat in D&D 5e shakes out, AC is frequently not that important a feature for determining tactics. The usual decision is between using Actions for things with attack rolls versus using Actions that force saving throws, though of course the DM can vary things.

In that light (pun not quite intended), I'll ask this regarding using poor lighting as cover: what tactical possibilities do you believe arbitrarily raising AC will open up, and do you have plans for other mechanical applications of lighting conditions beyond just raising AC?

The more thorough and well-planned yes answers you have to those questions, the more reasonable adding a homebrew mechanic around poor lighting adding AC seems to me. I'm not necessarily endorsing that approach, but examining those questions is how I would approach the possibility.


Lighting conditions and Disadvantage

Manipulating light is not very hard to do in most cases in 5e: the Darkness spell trivially gives every advantage darkness offers, and a variety of easily available items (like torches) and common spells that don't drain limited resources (like Light) fix various types of darkness. This could offer new tactical possibilities, like having a player dedicated to battlefield control by cleverly moving lights around, even with the RAW lighting rules. If the benefits of battlefield light management are too strong, however, then many players will essentially never not use them. And you'll have to balance combats for the extra advantages of those tactics, so players will effectively penalized for not managing light.

Advantage and Disadvantage are significant enough that they are worth getting/avoiding when convenient, but are not always worth a huge resource expenditure to obtain: spending a turn solely to gain Advantage on the next turn's attack is generally worse than just attacking once on each turn. Bonus AC is a different story, and is often much more valuable than Advantage/Disadvantage. One PC, trivially and reliably offering AC bonuses of +2, +5, and +∞ to the party while denying those to opponents, seems to me to be too strong an incentive. Advantage and Disadvantage, as more modest benefits, can be situationally great choices but not so amazing that players must employ strategies to get them at every possible opportunity.


How I would respond to your proposed lighting systems as a player

Light management would boil down to two situations for me: ambient bright light, and ambient darkness.

In ambient bright light, I would cast (or otherwise establish) Darkness and maintain it at all times, enveloping my party. This would give us easily moveable cover, granting an extra 2, 5 or infinite AC (depending on how much cover you rule it would offer). Additional castings of Darkness, with other characters, makes the approach even better.

If you tried to limit the cover bonus to dim lighting only, I would probably complain-- why would dim lighting provide more of the benefit than total darkness? This situation could get pretty close to a "never lose" setup. There would be very few situations in which light management is possible but this particular tactic is not. This might be a strongly dominant strategy (though I haven't run the numbers on it), meaning that doing otherwise is always a worse choice. That is the opposite of tactical variety.

In ambient darkness, I would employ creatures with low combat value (familiars, charmed beasts, hirelings, etc.) to move around with torches, items under effects of the Light spell, and similar. Using preplanned tactics, shouted utterances, and Readied Actions/Movement I would work to ensure that enemies are always in bright light and that the PCs never are. If there is an AC bonus to being in dim light, that only makes the tactics more flexible because less movement is required to gain or remove lighting-derived AC bonuses.

Cover bonuses for poor lighting seem to me to be so good that you won't see PCs ignoring these applications any more often than you see a low INT Wizard or low CON Barbarian. Conversely, Advantage and Disadvantage would be more likely to provide emergently beneficial situations (given where we are and what the battlefield is like, can I maneuver the enemy such that I'm in dim light while they're in bright light?), but are not so good that I would make a point of using spell slots and turns to set them up for myself and my whole party. It may still be a good tactic, but is not so dominant.


What I've done with light and darkness

In combats I use darkness almost exclusively for a couple of purposes: to restrict movement (a PC doesn't know what they might run into, and so may be more cautious) and/or to give enemies an edge in tactical positioning. I like the idea of more effects, but lighting is not a very rich mechanic in 5e and so I have found those to be best when limited to gimmicks in individual fights rather than new rule systems.

PCs with less effective room to maneuver tend to view the battle differently than if moving were easier. Enemies in the darkness can use the Hide action more effectively than would otherwise be possible, and if hidden they get Advantage on attack rolls. In my experience, those situations cause players to think up and use different tactics than they favor in well-lit situations.

True, those tactics often involve fixing the lighting through various means, but using one or more turns to do that can still be an interesting choice. Characters with Darkvision are supposed to be extra-effective in poor lighting, and nerfing that because you think lighting can be a more interesting mechanic is often viewed as poor form for a DM. There are opportunity costs to having Darkvision, and so the benefit should exist if a player chooses that trait. It would be similarly inappropriate to arbitrarily inhibit an elf's resistance to being charmed or immunity to being magically put to sleep just because you want to deploy tactics based on those effects.

Example in practice: I had four PCs in a pitch-dark crevasse with limited floor space. The first things they did were to throw down a torch and items with Light cast on them, creating two bubbles of light they didn't dare step out of. Meanwhile, a group of Slithering Trackers snuck around outside of those bubbles, hiding effectively, and were able to strike from unexpected angles and at unexpected times. They ultimately split the party between the two light bubbles and nearly achieved a TPK.

It ended up being an exciting, harrowing, and memorable experience for the players because of the mood of the situation itself. The fight would have been very different if it had been focused around knowing where the enemies were more of the time but having more trouble hitting them. The extra turns even a marginal increase in AC would have granted would almost certainly have led to a TPK (though, had I used higher average AC per round, I would have prepared the combat differently). Even if a PC had had Darkvision, that wouldn't have helped the other players very much, especially after I succeeded in splitting the party.

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  • \$\begingroup\$ Thorough answer! You mention potentially using Disadvantage to achieve the effect OP wants in dim light - would you be able to briefly talk about the implications of that option, i.e. it reduces the difference between dim light and darkness in combat? \$\endgroup\$ – Vigil Dec 3 '20 at 10:35
  • \$\begingroup\$ @Vigil I took a shot at fleshing that section out, but it came out a lot more ramble-y than I'd hoped. \$\endgroup\$ – Upper_Case Dec 3 '20 at 21:37
  • \$\begingroup\$ @Upper_Case Thanks! Very thorough and convincing. It's interesting to see what your approach as a PC would be, too. I think playing on perception (possible traps), and finding a way to discourage all party members to have Darkvision (disadvantages in other situation), will produce a better result than trying to find mechanical work-arounds, indeed. \$\endgroup\$ – RedGlyph Dec 5 '20 at 14:31
  • \$\begingroup\$ I agree with all of the answer, except the way it seems to present Advantage as not so important. Statisticallly Advantage is pretty much as strong as getting +5 to hit. Advantage is that strong. Thus all the tactics you list remain valid "as is" (with lighting affecting only Advantage), with the one "limitation" being that there are other more interesting and easier ways to grant Advantage. I definitely agree that making lighting give cover is probably not such a great idea. \$\endgroup\$ – Pat Feb 3 at 15:09
  • \$\begingroup\$ @Pat I did not mean to give the impression that Advantage isn't worthwhile, only to point out that it is (in many, perhaps most, situations) not worth using an Action to achieve for a single attack on the next turn when you could simply Attack on both turns, especially when multiple attacks per action are available. You can vary AC and to-hit bonuses to find edge cases, but frequently you're better off with two attacks (especially from an average damage perspective). \$\endgroup\$ – Upper_Case Feb 3 at 16:39
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Dim light is not cover

A lightly obscured area imposes disadvantage on Wisdom (Perception) checks that rely on sight. That's the only effect it has rules as written:

A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.

PHB page 183, "Vision and Light"

On the other hand, Cover is a completely different thing — it is something that, well, covers at least 1/2 of the creature and provides substantial protection:

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm.

...

A target with half cover has a +2 bonus to AC and Dexterity saving throws.

D&D 5e doesn't pay attention to light conditions much. If this aspect should be important in your games, you might consider using another game system (a random example is Torchbearer).

I advise against introducing a house rule for this

Rules have costs. When you implement a new rule, you have to work it out, you have to write it down and edit it for clarify, you have to present it to the table and ensure everyone’s understanding of the rule, and then you have to devote game time to enforcing it.

In this particular case you have to solve a few problems which might emerge. You probably should manage the Dexterity saving throws separately, otherwise there are silly situations possible when you can dodge AoE more effectively by being seen poorly by the caster. In a large light obscured area cover becomes useless — this works against tactics, since using cover becomes irrelevant (cover do not stack in 5e). In a uneven-lit area you have to constantly re-calculate AC, et cetera.

5e tends not to re-calculate AC at all, except for the Cover case. But Cover works for all creatures the same way, while "dim light cover" works presumably only for creatures without Darkvision. This imposes additional calculations (and calculation mistakes) at the table, which is not a good thing.

Darkvision is already quite powerful, I've seen players who choose their race (Elf or Dwarf instead of Human) just because of it. Being an ultimate counter to the "dim light cover" makes other choices even more sub-optimal.

All games, by necessity, skip over some of the details of real life; this is known as abstraction. And the choice of abstraction is a very important design consideration for a game—and never a trivial one. There are always trade-offs.

It will be okay as a ruling

5e tries to resurrect the "rulings over rules" principle from the old editions. As a DM you can give (dis-)advantage depending on the situation:

The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.

PHB page 173, Advantage and Disadvantage

If you think it is reasonable for light conditions to impose disadvantage in this particular situation, you are free to do that. Like, "bright light from the lamp blinds goblins, so they attack with disadvantage". Writing a whole new rule for this is unnecessary.

If your players want more tactical options, don't give them more rules, reward their cleverness and ingenuity instead. Let them explore and use things in the game world: environment features, expendable items, enemy's weaknesses, tactical moves (ambush, formation), social interactions, creative using of spells, you name it.

As a middle way you might introduce small heavily obscured areas in the dungeon. Being lit with a torch, they becomes lightly obscured, but without any illumination a creature in such an area becomes unseen.

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  • \$\begingroup\$ Thank you for your answer, it clarifies the situation very well! Indeed, each modification is a risk of breaking the balance in so many places. It does feel like an omission though: if one has a disadvantage spotting a creature in dim light, they should also have difficulties hitting them as efficiently as in plain light. Perhaps I can choose a few particular spots and consider them Disadvantage, as you mention. This would still have to be clear for the players, though. \$\endgroup\$ – RedGlyph Dec 2 '20 at 12:44
  • \$\begingroup\$ Dim light and darkness can provide the conditions of Lightly Obscured and Heavily Obscured for creatures that don't have ways to mitigate them (darkvision, torches, etc.). This is already covered in the rules. Having them provide cover in addition would be very strange, even as a house rule. \$\endgroup\$ – RevanantBacon Dec 2 '20 at 18:22
  • \$\begingroup\$ @RevenantBacon rules as written, creatures in lightly obscured areas have disadvantage on Wisdom (Perception) checks. This is irrelevant in combat, unless the foe is hiding. The OP wants more tactical diversity in combat, in terms of different light conditions. \$\endgroup\$ – enkryptor Dec 2 '20 at 19:34
  • \$\begingroup\$ @enkryptor Any creature can attempt to hide as an action, and rogues can do so as a bonus action. It's perfectly relevant in combat. And why should it be more relevant in combat? Hiding is a tactical option, just because it's not a tactic that they want to use doesn't mean they need to come up with a different use for mechanics that facilitate it. \$\endgroup\$ – RevanantBacon Dec 2 '20 at 19:37
  • \$\begingroup\$ @RevenantBacon this probably worth its own question, but AFAIK you can not hide being in a plain sight (unless a feature says so), see PHB p. 177 "Hiding". Just being in a lightly obscured area is not enough. \$\endgroup\$ – enkryptor Dec 3 '20 at 10:21
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It would not be "correct" (i.e. RAW)

Cover (half/three-quarters/total) as described in "Cover [PHB p.196]" and Obscurement (light/heavy) as described in "Vision and Light [PHB p. 183]" are two distinct systems that have different rules. These rulesets can interact with each other (e.g. you can both be in darkness and have half cover) but are not intended to replace each other - or they would say so.

The only generic RAW mechanical consequence of being lightly obscured/in dim light (i.e. other than certain specific race/class/monster features that key off it) is disadvantage on sight-based Wisdom(Perception) checks.

Is this a problem?

It depends on your perspective - but I can certainly see where you are coming from in thinking that it is and asking this question. Darkness is a common element in adventure storytelling - stalking through dark caves fearful of what hides in the shadows, all the lights in a room suddenly guttering out and being able to see nothing, etc. A party full of darkvision-capable heroes mitigates this a lot - indeed, as you note, in combat terms there is almost no downside to fighting in darkness, so long as the enemies are within their darkvision range.

How can it be solved?

There are several options I can see to make darkness play more of a strategic role in combat - some house-ruling, some leaning on game elements already within the rules. I'll present a few that I can think of.

Make use of monsters that have features utilising darkness/dim light

Some monsters (e.g. Boneclaws, MToF p. 121) have features that allow them to use areas of darkness/dim light to their advantage. Such monsters are built to be able to use such areas strategically, regardless of the darkvision of the party.

Leverage the mixed nature of a party

Some of the PCs in a party will likely have darkvision. But some may not, and they will not like fighting blind - so will probably insist on torches or magical light. If some of the party can be put in danger by darkness, by extension the whole party can too as the effectiveness of those PCs decreases.

Use darkness beyond vision range

Most PC darkvision only goes 60 feet. Monsters can lurk or attack from further than that and be cloaked by darkness regardless of PC darkvision.

Use magical darkness

Darkvision does not see through magical darkness, so all PCs are equal(ly blind) within it.

Restrict PC races or remove/modify darkvision

If darkvision on PCs does not fit the tone you want your game to have, you can restrict which races players can pick to those without it, or (in my opinion preferably) remove darkvision from the races that have it and replace it with a commensurate feature to compensate. You could also edit the feature so that it is more limited, perhaps only allowing improved vision in dim light rather than also in darkness - this too would require some compensation to balance it.

The optional Custom Lineage rule [Tasha's Cauldron of Everything p. 8] allows players creating a custom race to pick either darkvision (60ft.) or a skill proficiency - so the game designers consider an unrestricted skill proficiency to be about equivalent.

Add more mechanical consequences to dim light

If your main issue is that there aren't many downsides to fighting in dim light (or darkvision PCs fighting in darkness), then you could add some further mechanical consequences to dim light/light obscurement. Exactly what these would be would depend on what you perceive that environmental factor to be currently missing. If you do this, you should be cautious of the interactions and potential counterintuitive effects - e.g. if you give creatures in dim light half cover, does that mean hiding behind a rock that covers half their body gives no further benefit? Do they lose that half cover in darkness, since the rule only applied to dim light, and just have the disadvantage to attacks on them?

Conclusion

Dim light is not the same as cover. If you have an issue with the mechanics around it, think carefully about what your issue actually is before changing the rules - there may be something in the existing rules that helps, and even if there is not an ill-conceived house rule can have unintended consequences to the balance and verisimilitude of the game.

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    \$\begingroup\$ I especially like your suggestion of removing/restricting Darkvision. It might be worth mentioning how Tasha's Custom Lineage racial option considers Darkvision to be roughly equal to a skill proficiency. \$\endgroup\$ – user60913 Dec 2 '20 at 11:28
  • \$\begingroup\$ I am the opposite to Odo here and I don't like the suggestion. I would be very careful with it because as a player it would immediately tell me that you are wanting to punish being in th dark. A DM has enough tools to make the players face danger without changing rules to disadvantage them. \$\endgroup\$ – SeriousBri Dec 2 '20 at 11:36
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    \$\begingroup\$ I suspect we are coming from different types of gaming experiences. I don't think there are any concerns about disadvantaging players relative to the DM because the two aren't competing and the DM will still be responsible for creating balanced, enjoyable encounters. Restricting darkvision helps to create a different feel in the game in the same way as the Slow Natural Healing rule variant from the DMG or the Resurrection Madness from CoS. \$\endgroup\$ – user60913 Dec 2 '20 at 11:56
  • \$\begingroup\$ Thanks for your thorough answer! Hmm. Many races have Darkvision, and parties having mostly/only Darkvision races would not see the difference. If I want them to be clever about how they must tackle the dark parts, I have to find an artefact. The 60-foot limit won't do, it's a huge distance. Forbidding Darkvision is too limitating, Darkvision could be interesting in other areas. Magical Darkness might work, but it's artificial... The advantages monsters can have in darkness is interesting! Or other areas could have a disadvantage to races with Darkvision, but in practice it's not obvious. \$\endgroup\$ – RedGlyph Dec 2 '20 at 12:32
  • \$\begingroup\$ @RedGlyph I don't allow Darkvision to function while they are in bright light. So having PCs without it and requiring a light source limits those with Darkvision a bit. Also, they can't see color with it which some traps require you to see that. \$\endgroup\$ – Slagmoth Dec 2 '20 at 13:19

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