How are features that automatically deal damage to be balanced?

Question

There are a few spells and other features that can automatically deal damage, but it's very rare. The most common example I can think of is magic missile, which strikes unerringly and deals its damage with no attack roll or saving throw involved. Some area effect spells like cloud of daggers also deal their damage with no attack roll or saving throw.

In these particular examples, I see mitigating factors: for magic missile, the property of the shield spell to negate magic missile provides a way to counter it and the low damage dice seem to trade damage magnitude for reliability; and for cloud of daggers, a creature usually has to choose to move into the 5 foot area of the spell to take the damage, so it's mostly good for choke points.

However, in these particular examples, I also see counterpoints to those mitigating factors: very few targets are going to have the shield spell, and magic missile deals force damage, the least likely type for a creature to resist or negate; and cloud of daggers can be cast right on top of a creature to deal its initial damage without any practical way to be avoided.

Now, these two spells are just examples of the kinds of spells and features I'm talking about. There is advice in the Dungeon Master's Guide for designing new spells (p. 283 to 284), but it's very brief and doesn't seem to address this issue. My question is: How are these sorts of features (those that automatically deal damage) to be balanced relative to other features of a comparable level or categorization, based on official guidance or an analysis of the apparent techniques used in official material?

An alternative way to phrase the question with (as far as I can tell) an identical meaning is: "When is it balanced to not make an attack roll or require a saving throw to deal damage with a feature?" (In case I'm exhibiting the XY problem, my intent is to make a monster feature that automatically deals damage without violating the balance of the game, without simply copying magic missile, and without limiting my understanding of the balance issues to a damage-per-round calculation in consultation with the monster statistics table in the DMG. I would like to know how to balance such a feature without assuming it's for use by a monster.)

Answer 1

Spells can be balanced by not being useful in all situations

You mentioned two examples that are actually perfect examples of balancing. Cloud of Daggers is balanced because it fills such a small space. In most fights, you wouldn't use it because it's so easily avoidable except for the first cast. For a 2nd level spell, 4d4 isn't a whole lot of damage (8-12 average), and the area is inconsequential. BUT, in the right circumstance, it is very powerful. Similarly, if you want automatic damage, you need to make sure your spell is useful in a specific situation and not all situations. See also my answer about Stinking Cloud's effectiveness.

Spells can be balanced by the numbers

Let's compare Magic Missile (3d4 + 3) to a few other 1st level spells, and we can see how each are balanced. MM is a good comparer because it does away with the saves, so it's a good example of how much damage a 1st level spell should do at the base level. As you add modifiers, you can fiddle with the numbers. When you add something positive, you take away damage. When you add something negative (making a save, making an attack roll), you add damage. That's the general principal behind all game balancing.

• Magic Missiles: mean 10.5 damage. Hits automatically, can split damage

• Burning Hands: We start with the AoE effect. Since it can hit more than 1 creature, we reduce the damage some. But, we also add a save to the targets, which offsets the negated damage and allows us to configure the spell with more up front damage. The mean damage without saves is 10.5 to each creature (before saves), same as magic missile. Burning hands is a much better spell in the right circumstances and has the added bonus of igniting things. It also has some guaranteed damage because targets take at least half, unless they have feats that allow them to negate this.

• Guiding Bolt: Makes an attack roll, but does a mean of 14 (4d6) damage. Also adds advantage to the next attack against the creature it hits, which means we take away some damage. Also has a long range, so some damage is taken away in the design process.

• Inflict Wounds: Make a melee spell attack roll. Making it melee makes it harder to use since the range is limited. That's a negative, so we add damage. The attack roll is also a negative, so we add damage. Mean damage 16.5 (3d10).

• Hunter's Mark: No saves is a positive, so we take away potential damage. Lasts a long time is a positive, transferable is a positive. This spell is balanced at 1d6 extra damage to attacks (mean 3.5).

So while all these spells are balanced, they have different functions. Hunter's Mark is a very efficient spell. One spell slot can last a long time and potentially add up to way more damage than a magic missile (3 hits is the same damage as 3 missiles). Burning Hands can inflict way more damage by hitting more than 1 target. Guiding bolt does pretty good damage, but allows another character to hit more frequently -- Perhaps a Great Weapon Master Paladin. Inflict Wounds is probably the purest form of damage for a 1st level spell, so you can use that as a sort of base line. I would also assume guiding bolt's damage is also comparable to a pure damage 1st level ranged spell.

I would say there are a few simple examples of how you could balance a 1st level spell for automatic damage

• Short range: A spell that automatically dealt damage at a shorter range than magic missile could have it's damage increased to 5d4 (mean 12.5) or 3d6 (mean 10.5 with greater potential).

• Area of Effect: If you wanted to mimic cloud of daggers but increase to something like a 10 foot cube, I think the damage you're looking at is 2d4

• Persistent: If you wanted a spell that cost 1 slot but had a persisting effect like a damage over time, I think a 1d4 or a spell ability modifier to damage would be appropriate.

Basically, reference other similar spells and their damage and add damage for negative things and remove damage for positive things.

Answer 2

Magic Missile, at level 1, and Power Word: Kill, at level 9, are good case studies for extrapolating what homebrew spells should hew to.

Magic Missile (using a level 1 slot) deals 3d4 + 3 damage, which evaluates to an average of 10.5 damage, with the ability to roughly split up that damage among 3 enemies. Now compare against other level 1 spells:

• Witch Bolt deals 65 damage over 10 rounds, but requires a successful ranged spell hit AND that concentration + a consumed action be consumed for each of those rounds (which means forgoing any other damaging action during those rounds). Note that crits can deal double damage
• Chaos Bolt deals 12.5 damage, with the potential to deal more damage to a different target on a lucky roll. Note that crits can deal double damage
• Burning Hands deals 10.5 damage to a group of creatures, but requires a dexterity saving throw or the damage gets reduced to 5.75.
• Thunderwave deals 9 damage to a large group of creatures and causes a knockback effect, requiring a Constitution Check to negate the knockback and halve the damage.

The pattern is pretty clear: Magic Missile deals, arguably, the lowest amount of damage of any of those spells, on a single use of a first-level spell slot. However, that damage is guaranteed. If you cast Magic Missile on an opponent that can't cast Shield, you're guaranteed a minimum of 6 damage, with the potential to deal up to 15. None of those other spells guarantee at least 6 damage on a single cast.

So the rule, for any given level of your "unerring fire blast" or whatever spell, is that it should probably deal less damage than any other spell in its equivalent spell level, with the reciprocating advantage being that you don't need to worry that it won't discharge.

As for what that damage should be, that's why I mentioned Power Word: Kill. Because we're given two extremes of the spectrum: at level 1, a [nearly] unblockable damage spell deals about 10 damage. At level 9, a [nearly] unblockable damage spell deals about 100 damage (presuming we treat "instant death when below 100 hitpoints" as effectively equivalent to dealing 100 hitpoints worth of damage). So in your situation, I'd try to scale between those two extremes.

Personally, I'd try to scale quadratically, for the simple reason that low level spell slots are far more common and recoverable than high level spell slots (almost every class has some kind of mechanism to recover used-up SL1-5, whereas very few classes have any such mechanism for 6-9). So I'd recommend the following chart for calculating the average + median damage that a spell like this should be capable of dealing:

• SL1: 10 damage
• SL2: 15 damage
• SL3: 20 damage
• SL4: 25 damage
• SL5: 35 damage
• SL6: 50 damage
• SL7: 65 damage
• SL8: 80 damage
• SL9: 100 damage

You'll also observe that most of these effects either don't have a damage type (PW:K effectively deals 100 damage, but it doesn't have a literal damage type), or else deal Force Damage. That's telling. Force Damage is one of those "universal" damage types, in that almost no creatures have resistance or vulnerability to it, so if you're looking for guaranteed damage, it's extremely likely to land without being broken in either the caster or the target's favor.

For creature/class features, the balancing is a little more tricky, but I'd just borrow the CR calculator, use the average damage as the damage factor when calculating CR, and then scale until you achieve the desired CR.