Aku, Ascended

Legendary Resistance (5/Day). If the lich fails a saving throw, it can choose to succeed instead.

God Lich Aura. Aku is surrounded by a 10ft aura of un-death, any creature that enters this aura for the first time must make a constitution saving throw (DC 16) or take 2D8 necrotic damage, the damage taken is the siphoned to Aku and he gains hit points equal to the damage taken. If a creature saves on this effect it is immune to the effect for 2D4 hours

Magic Resistance.  Aku has advantage on saving throws against spells and other magical effects, and cannot be affected by spell effects cast at 4th level or lower. 

Rejuvenation. Aku has 4 Phylacterys split in to humanoid souls. if Aku's current body is destroyed while one or more of the vessels still lives, he can choose any Phylactery and consume its soul and and emerge from the humanoid with all his hit points and spell slots. If all Phylacterys are destroyed Aku can perform a ritual on a new humaoid vessel (willing or un-willing) to create a new living Phylactery.

Innate Spellcasting. (Psionics) The Lich's innate spellcasting ability is Intelligence (spell save 21). It can innately cast the following spells, requiring now components.

At will: detect thoughts, levitate 1/day: dominate monster, Plane shift (self)

Spellcasting. The lich is an 12th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 21, +13 to hit with spell attacks). The lich has the following wizard spells prepared:

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

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

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 (1 slot): disintegrate, globe of invulnerability

7th level (1 slot): finger of death, plane shift

8th level (1 slot): dominate monster, 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.

The Book Of Vile Darkness. Aku is in possession of The Book Of Vile Darkness and gains the following ability's from it:

Mark of Darkness. After you spend the requisite amount of time reading and studying the book, you acquire a physical disfigurement as a hideous sign of your devotion to vile darkness. An evil rune might appear on your face, your eyes might become glossy black, or horns might sprout from your forehead. Or you might become wizened and hideous, lose all facial features, gain a forked tongue, or some other feature the DM chooses. The mark of darkness grants you advantage on Charisma (persuasion) checks made to interact with evil creatures and Charisma (Intimidation) checks made to interact with non-evil creatures.

Command Evil. While you are attuned to the book and holding it, you can use an action to cast the dominate monster spell on an evil target (save DC 18). You can’t use this property again until the next dawn.

Dark Lore. You can reference the Book of Vile Darkness whenever you make an Intelligence check to recall information about some aspect of evil, such as lore about demons. When you do so, double your proficiency bonus on that check.

Dark Speech. While you carry the Book of Vile Darkness and are attuned to it, you can use an action to recite words from its pages in a foul language known as Dark Speech. Each time you do so, you take 1d12 psychic damage, and each non-evil creature within 15 feet of you takes 3d6 psychic damage.

Tentacles. Melee Weapon attack: +13 to hit, reach 5ft, one creature, Hit: 31 (4d10 + 5) psychic damage. If the target is Large or Smaller it is grappled (escape DC 17) and must succeed on a DC intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon attack: +13 to hit, reach 5ft, one incapacitated humanoid grappled by the Lich, Hit: 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the lich kills the target by extracting and devouring it's brain.

Mind blast (recharge 5-6). The lich magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 18 Intelligence saving throw or take 27 (5d8 + 5) psychic damage and be stunned for one minute. A creature can repeat the saving throw at the end of its turns, ending the effect on a success.

The lich can take 5 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 lich regains spent legendary actions at the start of its turn.

Cantrip. The lich casts a cantrip.

Tentacles. The lich makes one attack with its tentacles.

Extract Brain (Costs 2 actions). The lich uses extract brain.

Mind blast (Costs 3 actions). The lich recharges its Mind Blast and uses it.

Cast Spell (Costs 1-3 actions) The lich uses a spell slot to cast a 1st-, 2nd-, or 3rd-level spell it has prepared. doing so costs 1 legendary action per spell slot.