Reading the PHB, DM, and MM, I am having a hard time understanding how healing works for necrotic damage done by the attacks of various undead, such as the Vampire’s Bite attack on page 297 of the Monster Manual. I think what happens is not terribly well explained. It says:
[…] plus 10 (3d6) necrotic damage. The target’s hit point maximum is reduced by an amount equal to the necrotic damage taken, […]. The reduction lasts until the target finishes a long rest.
It seems like, after being bitten by a vampire, a PC can recover all hit points after a long rest, but Cure Wounds can’t restore these lost hit points sooner due to only being able to restore hit points up to the new (reduced) maximum. That seems really weird to me.
Can Cure Wounds or other magical healing (e.g., Lay On Hands) really not restore hit points lost to the vampire’s bite?
Reduction in maximum hit points (max HP) is essentially 5e’s replacement for the energy drains, level drains and negative levels of earlier editions.1 As such, it is not meant to be easily or quickly overcome, so low-level magic does not work. In fact, the 5e demilich’s legendary action is called “energy drain” while many undead in 5e (specters, wights, wraiths) have a “life drain” effect similar to the vampire’s bite. However, max HP reduction is not exclusive to vampires or the undead in 5e. Players can temporarily cause the same effect with the 6th-level Harm spell and other monsters have similar attacks that do not involve necrotic damage. The other max HP reducing attacks and their respective cures are:
blue slaad’s claw – unclear (involves disease), but maybe only a wish spell (MM, p. 276)
chasme’s proboscis attack – long rest or spell like greater restoration (MM, p. 57)
clay golem’s slam attack – greater restoration spell or similar magic (MM, p. 168)
demilich’s energy drain – greater restoration spell or similar magic (MM, p. 48)
mummy’s and mummy lord’s rotting fist attack – remove curse spell or other magic (MM, pp. 228-229)
night hag’s nightmare haunting – greater restoration spell or similar magic (MM, p. 178)
otyugh’s bite – daily saving throw (involves disease) (MM, p. 248)
succubus’/incubus’ draining kiss – long rest (MM, p. 285)
As listed above, the means of restoring max HP are specified in the description of the ability/effect that causes the reduction to max HP. This is almost always powerful magic (i.e., the 5th-level “greater restoration spell or similar magic”) or a long rest, the latter particularly for undead (specter, vampire, wight, wraith). However, a DM can interpret “similar magic” or house rule other powerful magic that can immediately restore max HP. For example, the blue slaad’s claw and otyugh’s bite create diseases that cause max HP reduction, so the 6th-level spell Heal may work since it “ends … any diseases.” Similarly, Heal washes its target in positive energy, so a DM could rule that it restores max HP to characters affected by the “life drain” of undead.
The energy drain mechanic in AD&D 2e was the loss of entire levels and everything that had come with each (e.g., HP, proficiencies, skills, spells, etc.) while 3e imposed a -1 penalty on all skill and ability checks, attack rolls, and saving throws, -1 prepared spell and spell slot and -5 max HP (-1 HD). There is an EN World forum thread that anecdotally discusses the psychological impact of level and max HP reduction on players.