How different cantrips of Warlock work in DnD 5e

How does toll the dead 5e work?

Toll the dead was a necromancy cantrip that drained the life force of creatures, especially ones who had been recently hurt.

You point at one creature you can see within range, and the sound of a dolorous bell fills the air around it for a moment. The target must succeed on a Wisdom saving throw or take 1d8 necrotic damage. If the target is missing any of its hit points, it instead takes 1d12 necrotic damage.

[5e] Necrotic damage and undead? ... Necrotic damage does not heal undead in 5th Edition. Regular healing can heal any creature unless there is an explicit trait or feature that says otherwise such as Cure Wounds which says it does not affect undead or constructs.

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.

I would say most things killed by necrotic damage will have the "withered corpse look" (skin covering bones with nothing in between, a few strands of white hair left on the head, etc.). Spells do what they say they do, unless the DM is agreeable.

Main thing about necrotic damage is that undead and constructs are generally immune.Necrotic damage withers matter and twists the soul. But it does not per default reduce HP maximum. It can also just be a “type” or generate other effects.