Black King

Shapechanger. The Black King can use its action to polymorph into a Medium humanoid it has seen. Any equipment it is wearing or carrying isn't transformed.

Amorphous. The Black King 's shadowy form can through a space as narrow as 6 inches wide without squeezing.

Dark Devotion. The Black King has advantage on saving throws against being charmed or frightened.

Limited Telepathy. The Black King can magically transmit simple messages and images to any creature within 120 feet of it that can understand a language. This form of telepathy doesn't allow the receiving creature to telepathically respond.

Dominion Crown. The Black King's crown gives him his shadowy form. If the Crown is damaged, he must succeed on a DC 18 Constitution Saving throw or be stunned on his next turn. On a natural 1 or 2, the Black King loses form entirely.

If another creature wears the crown, they must succeed on a DC 18 Constitution/Wisdom Saving throw 3 times. If the creature instead failed 3 times, they lose control of their body and are possessed by the Black King, keeping their own stats other than their Wisdom, Charisma, and Intelligence. The possessed creature will immediately attempt to make an escape and get somewhere safe. After 1d6 days, the creature completely loses their body and it is transformed into the dark, dust-like form of the Black King, gaining all stats seen here.

Spellcasting. The lich is an 18th-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, prestidigitation, thaumaturgy

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

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): Awaken, Creation

6th level (1 slot): disintegrate

7th level (1 slot): finger of death

8th level (1 slot): dominate monster, Dominate Person, power word stun

9th level (1 slot): Power Words

Power Words: Kill, Heal, Fall, Blind, Deafen, Explode

Multiattack. The Black king may make multiple attacks on one turn.

Slice. The Black King hardens part of his form and slices at a single target: +2 to hit, reach 10 ft.,  Hit: 14 (2d6) Slashing damage + 4 Necrotic damage. 

Slimshot. The Black Kings condenses a part of his form and shoots it out at high speeds!: +5 to hit, range 60/120 ft., # target. Hit: # (3d8) Piercing damage.

Punish. If a creature makes an attack and missies, the Black King can teleport to another creature within 60ft and make a slice attack on that creature, rolling at disadvantage.

Rebound. If a creature make a ranged attack and misses, the Black king absorbs whatever ammunition was fired at him, and will shoot it back at the attacker, rolling at disadvantage.

The Black King can take 2 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Black King regains spent legendary actions at the start of its turn.

Cantrip. The Black King casts a cantrip.
Teleport. The Black King magically teleports up to 40 feet to an unoccupied space it can see.