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:
Casting time: 1 Action
Range: 5 feet
Components: V, M (a weapon)
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?