Bezazel, Shadows of God

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

High Magic Invulnerability(Wish): Some time in the past, Bezazel granted himself immunity from 9th level spells through the use of a wish spell. The only  way this can be removed is through the use of another Wish spell to remove the event from history. This is the only manor in which WIsh can be used upon Bezazel. This ability also removed Bezazel's ability to cast Wish.

God of Magic (False). By controlling the body of the god of Magic, any spells Bezazel casts are cast as if one level higher than the spell slot consumed. Bezazel is also immune to spells of 5th level or lower, unless he wishes to be affected. (This is similar to the rakshasa ability to resist any spell lower than 7th level.)

God of Fire (False). By controlling the body of the god of Fire, whenever Bezazel is subjected to fire damage (including his own), he takes no damage and instead regains a number of hit points equal to the fire damage dealt. All fire damage inflicted by Bezazel to a target immune to fire is treated as resisted. All fire damage inflicted by Bezazel to a target resistant to fire is treated as if it it isn't resisted. 

Spellcasting. Bezazel is a 20th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 26, +16 to hit with spell attacks). Bezazel has all wizard & Warlock spells prepared. Some of these spells are listed below for convenience.

Cantrips (at will): mage hand, prestidigitation, ray of frost, Fire Bolt, Toll The Dead 

1st level (at will): detect magic, magic missile, shield, thunderwave

2nd level (at will): acid arrow, detect thoughts, invisibility, mirror image

3rd level (at will): animate dead, counterspell, dispel magic, fireball, Thunder step, Lightning Bolt

4th level (At will): blight, dimension door, phantasmal killer, Polymorph

5th level (5 slots): cloudkill, scrying, Wall of Force, hold monster, Steel Wind Strike, Negative Energy Flood

6th level (4 slot): disintegrate, globe of invulnerability, chain lightning, Investiture of Flame

7th level (4 slot): finger of death, Reverse Gravity, Delayed Blast Fireball, Forcecage, Crown of Stars

8th level (3 slot): dominate monster, power word stun, Mind Blank, Maddening Darkness, Feeblemind

9th level (3 slot): power word kill, Meteor Swarm, Invulnerability, Time Ravage

Cast Spell. Bezazel uses an action to cast a spell. 

Bezazel can take 10 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. Bezazel regains spent legendary actions at the start of its turn.

Cantrip. Bezazel casts a cantrip.

Cast Spell (Costs 2 Actions). Bezazel casts one of his at-will spells.

Cast Greater Spell (Costs 4 Actions). Bezazel casts a spell of 7th level or lower.

Deceitful Whispers (Costs 2 Actions).  A creature Bezazel targets must make a DC 20 Wisdom Saving Throw, or be forced to make an attack or cast a harmful cantrip against one of its allies within its range. 

Flow with Shadow (Costs 2 Actions). Bezazel becomes unseen and teleports to a location he can see within 30 feet of him. He becomes visible at the end of his next turn, or the next time he uses an action, whichever comes first. 

Conduit of Magic (False) (Costs no Actions - May only be used once per round). Bezazel may use any number of his remaining actions to cast a spell of that level or higher. If he does not have the required actions he may substitute 1d10 damage per spell level to make up the difference.