Annihalus the lich king

Legendary Resistance (3/Day). If the lich fails a saving throw, it can choose to succeed instead.

Rejuvenation. If it has a phylactery, a destroyed lich gains a new body in 1d10 days, regaining all its hit points and becoming active again. The new body appears within 5 feet of the phylactery.

Aura of rot. any creature that starts its turn within 15 ft of the lich must succeed on a DC 20 Con save or take 10(3d6) necrotic damage that reduces the hit point maximum .

Weakness of Life. The lich has a weakness to healing spells they cause damage equal to the amount of healing done. The Lich is destroyed forever if the spell true resurrection is cast upon him.

Undying. When the Lich drops to 0 hit points, his bones turn to dust and he is forced to try to flee to a new host. All living creatures within a 10 foot radius sphere must make a DC 19 Charisma saving throw. The Lich can then choose one creature that has failed the save and possess them. If the creature is a PC, then the character is now under the DM's control. While possessing a creature, the Lich's main goal is to find a Medium or Large creature of any good alignment, which he can then kill and inhabit the corpse of. After inhabiting the corpse for one year, the Lich regains all of his normal abilities. If, when the Lich dies, there are no creatures within the sphere, or all creatures succeed on their saving throw, the Lich remains as a pile of bone dust. He cannot move, speak, take legendary actions or actions other than to attempt to possess a living creature that moves within 10 feet of the remains.   

Life in death. The lich regains hit points equal to the amount of necrotic damage taken.

Shadow merge. The lich can forgo its movement on a turn to either teleport up to 30 ft. or use the shadows to execute a melee attack action on a creature within 30 ft.

Spellcasting. The lich is an 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). The lich has the following wizard spells prepared:

Cantrips (at will): mage hand, chill touch, ray of frost

1st level (4 slots): detect magic, magic missile, shield, thunderwave, command

2nd level (3 slots): acid arrow, detect thoughts, invisibility, mirror image

3rd level (3 slots): animate dead, counterspell, dispel magic, fireball

4th level (3 slots): blight, dimension door

5th level (3 slots): cloudkill, scrying

6th level (2 slots): disintegrate, globe of invulnerability

7th level (2 slots): finger of death, plane shift

8th level (2 slots): power word stun

9th level (1 slot): power word kill

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

Multiattack. The lich makes 2 attacks only 1 of which can be a touch attack.

Deaths grasp. Melee weapon attack: +12 to hit, reach 5 ft., one creature. On hit target is grappled (escape DC 16). Until this Grapple ends The lich cant grapple another target.

Paralyzing Touch. (recharge 3-5) Melee Spell Attack: +12 to hit, reach 5 ft., one creature. Hit: 10 (3d6) cold damage. The target must succeed on a DC 18 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. 

Astral touch. Melee spell attack +12 to hit, reach 5 ft., one creature. Hit: The target must succeed a DC 20 wisdom saving throw or be knocked unconscious as their astral form leaves their body till the end of their next turn or the next time their body takes damage.

Mind Control. (Recharge 6) Creatures not that are not using any form of mind protecting magic are susceptible to the lich's mind control. 1 creature that can hear the lich within a 30 foot radius must succeed on a DC 20 Charisma saving throw or have their mind surrendered to the lich for 1 minute. A creature mind controlled by the lich is charmed and can remake the save on the end of their turn.  If a creature's saving throw was successful or the effect ends for it, the creature is immune to the lich's mind control for the next 24 hours.

The lich can take 3 legendary actions, choosing from the options below. reach. the legendary action option can be used at a time and only at the end of another creature's turn. The lich regains spent legendary actions at the start of its turn.

Spell attack. The lich casts a spell of 3rd level or lower.

Frightening Gaze (Costs 2 Actions). The lich fixes its gaze on one creature it can see within 10 feet of it. The target must succeed on a DC 18 Wisdom saving throw against this magic or become frightened for 1 minute. The frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to the lich's gaze for the next 24 hours.

Death Breath( Costs 3 actions) only after grappled creatures turn. The Lich exudes the black touch of death.The Lich chooses one creature it has grappled. That creature must succeed on a DC 20 Constitution saving throw. On a failure, the creature takes 10d12 necrotic on a success the creature takes half damage and is immune to Death Breath for 24 hours. If a creature dies from this effect, its body turns to dust.