Abraxas

Master of Magic. Abraxas is a spellcaster of indeterminate level. He is able to able to cast any spell of 7th level or less, without the need for any material components, provided the spell does not involve removal of himself or a player to a different Plane (such as blink, plane shift or teleportation).

Rule Breaker. Abraxas is able to concentrate on two different spells at the same time, and has advantage on any Constitution check to maintain concentration if he suffers damage.

Quick Thinker. If Abraxas casts any spell that ordinarily is cast as a Bonus Action, he can use his Action to cast a second spell, without either spell needing to be a cantrip.

Spellcasting. Abraxas is a 20th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 19, +12 to hit with spell attacks). As well as any other 7th level or lower spells available to him, Abraxas has prepared:
Cantrips (at will): acid splash, sapping sting, mind sliver, chill touch
1st level (6 slots): inflict wounds, chaos bolt, magic missile, shield
2nd level (6 slots): hold person, scorching ray, mind spike, earthbind
3rd level (6 slots): counterspell, dispel magic, haste, slow, pulse wave, lightning bolt
4th level (5 slots): blight, fire shield, vitriolic sphere, freedom of movement
5th level (4 slots): cone of cold, dominate person, destructive wave, wall of force
6th level (3 slot): chain lightning, disintegrate, magic jar, flesh to stone
7th level (2 slots):  crown of stars, prismatic spray

Mace. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 25 (4d8+8) bludgeoning damage plus 14 ([2d12+1) force damage.

Abraxas can take 3 legendary actions, choosing from the options below. Only one legendary option can be used at a time and only at the end of another creature's turn. Abraxas regains spent legendary actions at the start of his turn.

Crow. Abraxas unleashes a fierce shriek, causing all creatures within 100ft to have disadvantage on their next attack roll.

Snake Legs. Abraxas makes two melee weapon attacks with his serpentine legs. Each attack roll has +8 to hit and a reach of 5ft on a single target. On a hit, the creature takes 17 (2d8 + 8) piercing damage plus 17 (2d8 +8) poison damage.

Unfair Advantage. (Costs 2 Actions). Abraxas casts a spell of his choice, up to 3rd level. This spellcasting cannot be interrupted by counterspell or similar effects, but can still provoke a melee attack if a player has the Mage Slayer feat or similar ability.