[RPG] take an amount of necrotic damage from a wraith that’s greater than the hit point maximum

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.

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.

These rules have two entirely separate triggers, and if a single attack satisfies both, then so be it.

The attack reduced the creature to 0 hit points, and it was a melee attack. So you can choose to knock the creature out; it's now unconscious and stable.

However, there was leftover damage from the attack equaling or exceeding the creature's maximum hitpoints. So it dies.

The end result is that the creature is dead. If you're worried about realism, this is a perfect outcome: If you're trying to knock someone out, but you hit them hard enough to kill them, they die, regardless of your intentions.