Why can't Cure Wounds heal necrotic damage done by powerful undead, but sleeping can?

Question

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?

Answer 1

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.

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1. 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.

Answer 2

As for what is Necrotic damage:

Necrotic damage, dealt by certain undead and a spell such as chill touch, withers matter and even the soul. (PHB, p. 196).

You can think of it as damage the corrupts flesh, matter, and even impacts the very soul of a creature. My suggestion would be to describe it as a quick acting rotting effect with the flesh blackening and withering along with a feeling akin to a strong sense of sadness or loss.

The different damage types interact with various attacks and defenses. A creature may be vulnerable to necrotic, resistant, immune, or have nothing special in regards to this type of damage.

Like the other types of damage, it comes into play as the rules specifics says it comes into play.

If you look on page 197 of the PHB, first column there is a general note on Healing. There is no mention of any special treatment of necrotic damage over any other type of damage, nor do any of the healing spells mention necrotic damage.

Specific abilities and spells can deal both necrotic damage and effect healing. However it is not a function of necrotic damage but the ability/spell itself. It will be stated specifically in the description what the effect will be. For example Chill Touch on page 221 states

On a hit, the target takes 1d8 necrotic damage, and it can’t regain hit points until the start of your next turn. Until then, the hand clings to the target.

while Circle of Death on the same page says

Each creature in that area must make a Constitution saving throw. A target takes 8d6 necrotic damage on a failed save, or half as much damage on a successful one.

Both deal necrotic damage but only Chill Touch has any impact on healing. Circle of Death just deals the damage.

Another example is the Vampire Bite in the Monster Manual (Vampire)

Bite (Bat or Vampire Form Only). Melee Weapon Attack: +9 to hit, reach 5 ft., one willing creature, or a creature that is grapp led by the vampire, incapacitated, or restrained. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way and then buried in the ground rises the following night as a vampire spawn under the vampire's control.

You will note that its ability to reduce a target's hit point maximum is spelled out in the attack's description. This is because the necrotic keyword is not sufficient to cause this effect. Instead it spells out how the attack does damage and reduces the hit point maximum, as well as how it is recovered from and what happens when HP goes to zero.

Unless this text or something similar to it is present in the description of the power, ability, or spell then necrotic damage is healed like any other damage.

The inability to heal this damage is contingent on the key word hit point maximum. You cannot heal beyond the maximum hit points. The decrease can only be countered by the method outlined in the text or a spell the effects maximum hit points like Greater Restoration.