Is this homebrew life-stealing melee cantrip unbalanced?

Question

I started thinking about a variant of a melee attack cantrip like booming blade that would allow you to regain some lost health. The cantrip I came up with is this:

Evocation cantrip
Casting time: 1 Action
Range: 5 feet
Components: V, M (a weapon)
Duration: Instantaneous

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails.

On a hit, the target suffers the attack's normal effects, plus 1d4 necrotic damage. The target makes a Constitution saving throw against your spell save DC. On a failure, you heal for an amount equal to the necrotic damage dealt. On a success, nothing else happens.

The spell's damage increases as you reach higher levels.

The necrotic damage increases by 1d4 when you reach 5th level, for a total of 2d4 damage. This damage increases by 1d4 at 11th level and again at 17th level.

The issue with this cantrip is that the DMG (p. 284) clearly says that a cantrip shouldn't offer healing.

So the question is if

• a low extra damage/healing output,
• a Constitution saving throw
• and the possible addendum of resolving the weapon attack damage first, as to prevent the PC from abusing it on wildlife and easy targets

are enough to make it balanced?

Answer 1

It's probably unbalanced.

Let's take a look at what other things allow you to gain hit points at first level:

• Cure Wounds (requires a spell slot and action, restores 1d8 + change).
• Healing Potion (costs 50 gp, requires an action, restores 2d4+2).
• Goodberry (requires a spell slot and action, restores 1 hp per action for 10 additional actions).
• Healing Word (requires a bonus action and spell slot, restores 1d4 + change).
• False Life (requires an action and spell slot, gives you 1d4 + change temporary hit points).
• A short rest (requires an hour of inactivity, costs your only hit die, restores about half your HP).

A cantrip that lets you use the same action to both make an attack and also heal up to 4 hp is certainly more powerful than any of these options for 1st level characters.

A cantrip that lets you heal for even 1 hp would be strictly superior to a Short Rest, so probably even that would unbalance the game a little.

As you've written this spell, it should be at least a 1st level spell.

Answer 2

Definitely unbalanced

I can only speculate as to designer intent, but the mentioned admonition from DMG is likely because healing should consume resources, putting a practical limit on the amount of healing that can be done each day. As such, there is unlikely to be any extenuating circumstance that might balance a cantrip (no resource use) that allows healing.

What you might be able to get away with is changing the healing component of your cantrip to Temporary Hit Points. Even getting temps so easily could be unbalanced (most other sources of temps are either spells (False Life) or class features (Dark One's Blessing, which also requires killing the target rather than just hitting it), so the spell is probably still unbalanced, though the Con save requirement might help. You might also add the limitation that the caster can only benefit from these temps once per creature damaged with the cantrip.

Further, note that the other weapon based cantrips (Booming Blade and Green Flame Blade) don't provide their additional damage until some other condition is met (adjacent targets or the target voluntarily moves). Guaranteed additional damage, even just a d4, might be unbalanced, even without the heal/temp component. As such the Con save should prevent the extra damage completely.

Two options would thus be:

On a hit, the creature must attempt a Con save. On a failed save, the creature takes 1d4 necrotic damage and you gain the same amount of temp HP. You may only gain this temp HP once per 24 hours from any given creature.

Or:

On a hit, the creature must attempt a Con save. If the creature fails the save, it takes 1d4 necrotic damage and, the target's soul is marked. If the creature dies within the next minute, you gain 1d4 temp HP. Each creature may only be marked once in this fashion, additional failed saves merely reset the one minute duration of the mark.

Either option restricts the benefit to once per creature (similar to DOB), and places the additional damage behind a condition (the Con save). The damage and temp HP could probably be raised to a d6 without significantly unbalancing the cantrip at this point. Since temp HP can't stack, the cantrip is more difficult to exploit, and the first option (immediate temps) is more powerful by far (allowing you to benefit multiple times per combat, similar to DOB).

Answer 3

The cantrip is too easily abused.

Even without comparing this to official cantrips, your suggested homebrew cantrip is unbalanced. The benefits (regaining hit points) stack with themselves, and the cantrip can be used indefinitely against any creature. What you have here is a "bag of rats" scenario, in which the player has a benefit-per-hit ability that they can easily exploit by hitting many weak creatures.

For example, suppose a level 1 wizard literally carries around a bag of rats. Each rat is a CR 0 creature with 10 AC, 1 hit point, and a -1 Constitution save bonus. For the sake of simplicity, let's assume each casting of the cantrip has a 50% chance to successfully kill a rat and gain +1d4 hit points.

In 1 minute (10 rounds), if the wizard casts this cantrip each round, then they can can regain about ~5d4 HP. This expends no materials, and heals more than a consumable healing potion. And in 5 minutes, the wizard can regain ~25d4 hit points, which is cheaper than spending hit dice during a short rest, even at higher levels.

When the wizard runs out of rats, they can find new ones, likely for a lower cost than a healing potion or rest at an inn. This simple cantrip can break the system's hit point economy.

Can it be balanced?

As with any "bag of rats" exploit, you can prevent abuse by making the benefits non-cumulative, such as granting temporary hit points that don't stack with themselves.

Furthermore, these temporary hit points would need a very short duration. Otherwise the cantrip could be used indefinitely while the caster has a valid target, effectively rerolling the temporary hit points until they get a favorable result; this would rival an at-will false life, or the "Fiendish Vigor" warlock invocation.

As long as the cantrip provides some form of healing, it's going to be iffy in terms of balance.

Answer 4

While unbalanced, a simple change in what type of health is 'restored' could restore some balance to the cantrip. Perhaps instead of permanent HP being restored the spell could restore temporary HP that lasts until the end of combat up to some maximum such as character level. I am guessing that the original intent was to open up the possibility of a 'drain tank' character. While they wouldn't be able to heal up to full they would still become an annoying frontline since they could have a decent part of incoming damage soaked up by the small shield.