Corpsetaker

Legendary Resistance. If Corpsetaker fails a saving throw, it can choose to succeed instead by spending a legendary action.

Corpsetaker. Corpsetaker is a parasite spirit, always looking for a host. When the inhabited body is killed, the essence of Corpsetaker flees the dying body and attempts to inhabit another. When disembodied it is incorporeal, gaining a fly speed of 45 and may move through any non-magical object or creature.  It may make a single Agonizing Touch attack on it's turn (or for a legendary action) and when it does so, the creature hit must make the wisdom save associated with the attack or become possessed.  A creature that fails this save may try again at the beginning of each of it's turns.  Any creature possessed in this way for more than 1 minute gains disadvantage on any further saving throws against possession.

For every round Corpsetaker is insubstantial and not in a host, it must make a DC15 Wisdom saving throw or be banished as if by the Banishment spell.  If banished in this way, Corpsetaker can only materialize back on the Prime Material plane after a week has passed, and only if then summoned from the abyss.  If slain, Corpsetaker abides by the same rules as that of a Demon or Devil.

Dispel Evil and Good would work to eject Corpsetaker from a host.

Shared Vitality. Corpsetaker shares vitality with the host body, giving it a pool of 200hp, replacing the bodies original health.  When reduced to 0hp, the essence of Corpsetaker must flee the dying body, becoming an insubstantial spirit.  The spirit form of Corpsetaker has 100hp.  If it gains a new host body, the body has a pool of 200hp but any damage done to the spirit carries over to the next time Corpsetaker is ejected from a host.

Spellcasting. Corpsetaker is a 15th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 19, +11 to hit with spell attacks). The Corpsetaker has the following wizard spells prepared:

Cantrips (at will): mage hand, prestidigitation, fire bolt, Acid Splash

1st level (5 slots): detect magic, magic missile, shield, Cause Fear

2nd level (4 slots): Crown of Madness, detect thoughts, invisibility, mirror image

3rd level (4 slots): animate dead, counterspell, dispel magic, Melf's Minute Meteors

4th level (3 slots): Wall of Fire, dimension door

5th level (3 slots): Animate Objects, Dominate Person

6th level (2 slot): Chain Lightning, globe of invulnerability

7th level (2 slot): Delayed Blast Fireball, power word pain

8th level (1 slot): Feeblemind, Maddening Darkness

Turn Resistance. The Corpsetaker has advantage on saving throws against any effect that turns undead.

Preparations. Corpsetaker is centuries old and knows a vast number of spells. It will cast Simulacrum, Contingency, and Create Undead during any down time if the previous spells are not already active.

The Simulacrum created does not have access to any spells higher than 5th level.

When going into battle, Corpsetaker will always plan for an ambush and ensure it already has it's defensive spells active.  Frequently, it will take the body of a loved one to get close to a target.

Agonizing Touch. Melee Spell Attack: +11 to hit, reach 5 ft., one creature. Hit: 10 (3d6) psychic damage. The target must succeed on a DC 17 Wisdom saving throw or have their hit points reduced by the amount equal to the damage taken. Only a Greater Restoration or Wish Spell can undo this.

Corpsetaker can take 3 legendary actions, choosing from the options below. Multiple legendary actions can be used at a time but only at the end of another creature's turn. Corpsetaker regains spent legendary actions at the start of its turn.

Cantrip. Corpsetaker casts a cantrip.

Agonizing Touch. Corpsetaker uses its Agonizing Touch.

Spirit Jaunt. Corpsetaker uses the spell Misty Step requiring no spell slot.

Spellcasting (Costs 1 Action per spell level). Corpsetaker may cast a spell.

Mind Rend (Costs 3 Actions). Each non-undead creature within 20 feet of the Corpsetaker must make a DC 17 Wisdom saving throw against this magic, becoming confused as if targeted by a Confusion spell on a failed save.