Aku

Magical resistance: Aku has advantage on saving throws against magical effects. 

Demonic evasion: If Aku would pass a dexterity saving throw that would result in him taking half damage, he takes no damage instead. 

Demonic Shroud: Creatures attacking Aku Do so at disadvantage. This ability ends once Aku takes Damage from a to hit attack for the first time on a turn, and will recharge at the end of Aku`s turn. 

Lucky:  Aku has the Lucky feat ( 3 uses) 

Legendary resistance (4 uses): If Aku would fail a saving throw he can choose to pass it instead. 

Regeneration: At the beginning of his turn, Aku gains 3d10+10 Hitpoints. This trait is ignored if Aku has taken more than 50 radiant damage from one source. 

Shape changing: On initiative count 20 Aku is able to transform into one of the following creatures: Kraken, Ancient Black dragon, Demogorgon or Tarrasque. This Transformation can be treated as a Wild shape. While Transformed Aku loses all other special traits besides Spell casting, and gains all traits of the creature Aku Transformed into and remains in his new form until their Hit points are depleted (This includes legendary actions, lair actions ...ect). When Aku reverts to his original form he cannot transform into any creature he previously transformed into for 24 hours.  

FOOOLS: At the beginning of each players turn that creature makes a Wisdom saving throw (DC 21) or becomes Charmed by Aku. While Charmed this way the creature acts if they are frightened and use their turn to run away from aku. A creature remakes the save at the end of their turn. Once a creature passes this save they cannot be affected by this trait for 24 hours.

Spell casting:  Aku is a 20th level caster ( +15 to hit DC 22 spell save) and can cast any spell he so pleases excluding wish, and ignoring components,  and has slots as follows: 

cantrips: infinite 

1st: 4 slots (0 0 0 0) 

2nd: 3 slots (0 0 0)

3rd: 3 slots (0 0 0)

4th: 3 slots (0 0 0)

5th: 3 slots (0 0 0) 

6th: 2 slots (0 0)

7th: 2 slots (0 0) 

8th: 1 slot (0) 

9th:  1 slot (0) 

Multi attack: Aku makes 3 attacks, 2 slams, and one Demonic lazer attack.

Slam: Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) bludgeoning damage plus 11 (2d10) necrotic damage.

Demonic lazer: Ranged Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 20 (3d10 + 9) Necrotic damage plus 11 (3d10) force damage.