Why shouldn't one make changes to concentration in homebrew?

Question

It's not hard to find homebrew-review questions on this site that propose changes to how concentration works. Every time I have proposed such a homebrew, regardless of how limited the change to concentration is, I've been told that one should never change how concentration works. I've never found or been given a reason for this, however. Further, it seems strange to me that this would be so dogmatic, given that,

• The Arcane Abeyance ability lets another creature concentrate on a spell cast by a Chronurgy Wizard;
• Matt Mercer's homebrew Cleric Moon Domain allows dual concentration as long as both spells are from the class domain list (admittedly, Matt Mercer is not WotC, but he is cited frequently as an authority here with respect to rules and game design).

These all seem like substantial changes to concentration that are acceptable to the game designers. It is glaringly obvious that some changes to concentration will break the game or make keeping track of it substantially harder, but this is true of every feature in the game. It is not clear to me why this dogma about never changing concentration, specifically, gets repeated. What is the justification for this dogma and why are the above exceptions okay given the justification? Why does this dogma get repeated even when the changes to concentration are very limited?

I am aware, per this question, that the rules warn about this (this question is also related). Some answers to these questions give examples of overpowered concentration changes, but I don't find these very informative because they are so clearly overpowered. I am wondering what specifically breaks when concentration is changed and why the above examples don't also break that?

This question and answer address a specific very powerful change to concentration and has good advice for this question, overall. However, here I'm asking about about the boundary between game-breaking and non-game-breaking changes with respect to concentration.

Answer 1

I wouldn't say that "all changes to concentration are inherently overpowered", but I would say that all changes to concentration are inherently suspect. You'll need to playtest the heck out of it with players who are dead-set on abusing it to the utmost, and you'll still not be totally sure it's okay.

"Don't mess with concentration" is a great rule of thumb, the safer path that totally avoids a pitfall rather than trying to tightrope-walk across it.

If it feels like people on SE tend to have a kneejerk reaction to altering concentration rules, then this is probably the source of it. It's good advice to not mess with that part of the rules; but like all good advice, it's entirely possible to ignore it and still be fine -- it's just more dangerous. And more to the point, if you're asking "Hey what do you think of this class feature?", most people won't be able to look at a change to concentration and tell whether it's viable or not. Like messing with the action economy, the results can be far-ranging and unpredictable.

But it's not just concentration. Alterations to spellcasting rules in general are difficult to balance, because there are just so many spells and so many possible interactions that must be considered. On a monster whose lifespan is measured in a rounds (and usually a single digit) and has a limited spell list, a possibly unbalanced ability is not a big deal; worst-case, it throws off the difficulty of one fight. On a player character, it can have a much more devastating impact on the game.

Answer 2

It potentially affects every concentration spell...

...and how that interacts with every concentration spell, and how that interacts with every concentration spell, etc.

There are currently 237 concentration spells on D&D Beyond. Of those spells, 185 are 1st-5th level spells or cantrips, meaning a single character could have any two of them (via multiclassing). If you only want to check how any two of those 185 interact, you'd need to check at least 17020 two-spell combinations.¹ And that ignores high and low level combinations, like a Wizard 17 / Cleric 1 using shapechange and shield of faith.

And don't forget - that number increases every time WotC releases another book.

"But what if I restrict things?"

The important part is how you restrict things. The goal is to reduce the number of combinations you'll need to check. Here's some examples:

Choose one 1st level spell from your class' spell list. That spell no longer requires concentration for you.

There are 29 spells that qualify for that. Each would need to be tested against every other concentration spell in the game. That leaves 6467 combinations to test.²

Bless no longer requires concentration for you.

Bless would need to be tested against every concentration spell (including itself), leaving 237 combinations. That's still a lot, but way less than the less restrictive version.

As a Cheese Domain cleric, you can concentrate on two spells as long as both spells are from your domain list.

If all 10 of the Cheese Domain spells normally require concentration, then they'd need to be tested against each other. That brings it down to 55 combinations.¹

If you are concentrating on bane, bless, or shield of faith, casting a different spell from that list does not cause you to lose concentration on the first. You may not have more than two of these spells active in this way. If you lose concentration, both spells end.

This only requires checking 3 combinations¹ - a far more reasonable number. Furthermore, that number doesn't increase as new spells come out.

Long-duration spells make things even more complicated

In the previous sections, we were just considering two-spell interactions. If you start a fight without any spells running, then it'll take two rounds (in most cases) for you to get two concentration spells going. However, spells with a duration longer than 1 minute are often cast before the fight even starts.

Allowing a 9th level cleric to cast shield of faith (10m duration) or spirit guardians (10m duration) without requiring concentration, for example, means that both spells plus holy weapon (1h duration) could easily be active before the cleric even opens the door! That's triple the amount of active spells for no action at all!

The designers specifically advise against it

The DMG says:

Beware of adding anything to your game that allows a character to concentrate on more than one effect at a time [...]. Rules and game elements that override the rules for concentration [...] can seriously unbalance or overcomplicate your game.

This isn't just a cop-out answer. There are very few features of the system that are called out in this way. That likely makes concentration a fundamental part of the design of the system.

The meta reasons

In my experience, newcomers to 5e are often frustrated by the concentration rules. They take damage and lose a spell, or they can't shield of faith the fighter without losing spirit guardians. They want to avoid that frustration by changing or removing concentration.

What I'm saying is - concentration changes are frequently (but not exclusively) suggested by less experienced players. When people say "don't change concentration", they usually mean "changing concentration requires extensive knowledge of the system; if you're asking non-specific questions about it, you probably don't have the knowledge required, and there's no way we can convey that in a post".

I'm not saying that reaction is right, but people are good at picking up on patterns, and this is one of them.

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¹ Calculated as a combination with repetition, which I think is the right math here. This accounts for bless + shield of faith being equivalent to shield of faith + bless.

² Calculated with a simple product (237 × 29), minus the duplicates. Each spell after the first has to check one less combination (since it would be a dupe), meaning the total number of dupes is the sum of numbers between 1 and 28: 402. That leaves 6873 − 402 = 6467.

Answer 3

There are several reasons

The main issue with ignoring concentration is not game balance. 3.5e and Pathfinder have no concentration and are balanced games. The issue is playability, keeping the game simpler, so it can flow.

The game mechanics expect single concentration

All the rules in the game are written with the assumption that you can only concentrate on one spell at a time. This means, when you break that rule, you will create situtations that are not covered by these other rules, and if they come up it will take you time to find answers. Even if they are not hard to answer, they will require you to come up with your own answers, and document and remember them, creating administrative overhead.

As this is also a fundamental change in how the rules work, you will not be able to call on the help of experts such as those on this site.

One example: What happens when you are concentrating on two spells, and get hit? Will you have to make two concentration checks? One jointly for both? Is it the same DC, or harder for additional spells?

The game slows down with more stuff you need to track

Every ongoing effect that you add that has a limited duration needs to be tracked -- is it still going on? As soon as you allow multiple concentration, you allow more spell effects that need to be administered.

This slows down the game and detracts from the fun of developing the story, by making you or your players spend more time on mechanics and bookkeeping.

We played without concentration and attunement in 3e and Pathfinder, and being "forced" to only have a select few items and a single ongoing spell in 5e has been one of the best changes to make the game more fun, at least in my experience.

Broken Window Theory

Once you are OK with such effects in prinicple, you open the doors for other homebrew with the same argument -- that single exception won't hurt. Before you know it you have several different homebrew races, classes and subclasses that all allow this in some form, and then someone combines them in a multiclassed abomination that can run them all.

Finding combinations that achieve the extreme in something is not uncommmon (see the various questions about maximum number of skills in one character build, maxium number of attacks, maximum number of giving the finger to someone...)

Answer 4

Bounded Accuracy

One of the main design goals of D&D 5e was to limit the power range between low and high level characters. Another was to make martial classes more competitive with spellcasters.

Compared to 3.5e, high-level characters have lots more hit points and lots more options on what to do than low-level characters but things like AC, saving throws are better but only slightly better.

Allowing characters to layer concentration spells can potentially break this paradigm. Concentration is therefore a core part of the balance within 5e. Like proficiency bonus, ability score caps, and limiting magic items to +3. You mess with any of that at your peril.

In any event Arcane Abeyance is not messing with concentration: it is effectively creating a short-lived spell scroll that can be used by anyone. And, Matt Mercer is making entertainment, not playing D&D.

However, you don’t need anyone’s permission to screw with your game if you really want to. If it breaks, it breaks.

Answer 5

Those specific abilities are balanced by their limiting factors.

As I explain in my answer in the related thread, multiple concentration is a very powerful ability because it allows one caster to employ more spells in one combat, so it's generally overpowered. As the Dungeon Master's Guide hints, it would let you bring more buff spells to a fight, and require you to track those buff spells, which are bad for balance and speed of play, respectively.

However, that's a general rule. The examples you give carefully limited in scope in order to minimize their power:

• The Arcane Abeyance feature allows a wizard to give someone else a spell to cast. It only works for spells of 4th level or lower, the spell must be decided ahead of time, and very importantly it can only be used once per encounter. It's also still using someone's concentration. If you give it to a non-caster, they probably take more melee hits than a caster would, and make more saves for concentration than a caster would. Ultimately, it's giving the party a single additional concentration spell for one fight, which may be very good, but it's a lot more balanced than the general case of allowing a caster to concentration on two spells.
• The Moon domain, from the sources I can find, only works for spells on the domain list, which are mostly illusion spells, rather than buffs or damage. It also only works when both spells are on the domain list. It costs you a use of Channel Divinity, and you make any Constitution saving throws to maintain concentration at disadvantage.

DMG p.263 doesn't say you can't allow concentration on multiple spells, only that it is generally overpowered to do so. It's not actually a hard rule, just a warning. If you can create an ability which affects concentration and is not overpowered in practice, even when used optimally, that's fine. It's just that there are a lot of opportunities to create overpowered content this way.

Answer 6

The problem is not that changing concentration always bad.

The problem with changing concentration is not that doing so is inherently bad, it's because changing such a far-reaching and widely-used mechanic is very easy to mess up. Concentration interacts with a lot of different parts of the game, so it's all to easy to forget about some spell combo, or some specific set of circumstances, or whatever that makes some choice far stronger (or weaker) than intended. A lot of people, when presented with the possibility of messing around with something like that, will take the attitude of "just don't do it" because that guarantees they won't mess it up, and that might be what you've run into.

Go ahead and make any given change you feel like in the games you run, it's your decision, but the philosophy of "don't change concentration" is there primarily as a guideline because the chances of accidentally forgetting one thing that becomes problematic later is very high. You can largely circumvent this problem by having a gentlemen's agreement with your players to not abuse overpowered stuff, but at the point where you're relying on that for balance you open yourself up to making a lot of other not-necessarily-balanced changes (which may be good or bad, depending on your personal opinion).