Bill Cipher

Change Shape. Bill can transform into any creature he has saw before. He keeps all of his stats except for his attacks and size. You cannot tell him apart from the actual monster except for his glowing eyes. This also includes changing size. If Bill is 2x larger then double his dmg. by 2, and so on.

Darkvision. Bill can see perfectly in the dark.

Trait swap. Bill can pick any two traits he wants, if he wants three, then he will have to swap it out for another that he already has. This action takes two turns.

Anger.  If Bill is down to half health, he casts Wish even if it is not his turn. This does not consume a spell slot.

Spellcasting. Bill is a 9th-level spellcaster. His spellcasting ability is Intelligence. The Bill Cipher has following Wizard, Warlock, and Sorcerer spells prepared:

Cantrips (at will): Minor illusion, Prestidigitation, Message, Mending, Eldritch Blast,                         Mage Hand, Blade Ward, Light, Dancing Lights, Shocking grasp.

1st level (15 slots): Detect Magic, Disguise Self, False Life, Ray of Sickness, Charm                         Person, Hellish Rebuke, Chromatic Orb, Sleep, Unseen Servant.

2nd level (12 slots): Alter Self, Blur, Invisibility, Crown of Madness, Ray of Enfeeblement, Enthrall, Shatter, See Invisibility.

3rd level (10 slots): Fear, Fireball, Vampiric Touch, Major Image, Hypnotic Pattern, Counterspell, Bestow Curse.

4th level (8 slots): Banishment, Arcane Eye, Dimension Door, Hallucinatory Terrain, Phantasmal Killer Confusion.

5th level (6 slots): Animate Objects, Contact Other Plane, Modify Memory, Legend Lore, Dream, Dominate Person.

6th level (6 slots): Arcane Gate, Circle of Death, Create Undead, Eyebite, Move Earth, True Seeing, Sunbeam.

7th level (5 slots): Plane Shift, Etherealness, Finger of Death, Reverse Gravity, Teleport

8th level (4 slots): Telepathy, Mind Blank, Demiplane, Clone, Feeblemind, Control Weather

9th level (3 slots): Weird, Wish, Time Stop, Foresight, Imprisonment, Astral Projection, Gate,  Meteor Swarm, True Resurrection

Snap. Roll 1d4, a 1 one is a cantrip, and so on. Pick a random spell (if possible, roll a die) and that is the attack. This does not consume a spell slot. The spell can be modified to where it can be applied when he is snapping.

Punch. Melee Weapon Attack: +5 to hit, reach  50 ft., 1 target. Hit: 17 (3d10 + 5) bludgeoning damage. If he is angered, his hand grows in size and can hit 30 targets.

Laser. Ranged Weapon Attack: +3 to hit, range 20/1000 ft., 1 target. Hit: 26 (4d8 + 10) fire damage. This attack shoots in a straight line, and burns a hole through the target if they drop to 0 hp because of this attack and attacks anyone who is behind them.

Absorb. Three times (at max) Bill can transport the attacker into a demiplane where time has stopped. This lasts for 3 turns, and the attacker doesn't remember anything about the demiplane.

Bill can use any Lengedary Action at any time throughout the game. He can only use them once, though.

Spellcast (Costs 1 Action). Bill casts a spell of choice.