Conquest

Plague Aura. Each creature within 20 ft of Conquest at the start of Conquest's turn must succeed a DC 18 Constitution Saving Throw or be poisoned until the end of conquest's next turn. 

Perfect Darkvision: Conquest can see normally in darkness, both magical and nonmagical.

Spellcasting. Conquest is a 20-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 21, +11 to hit with spell attacks). Conquest has following Wizard spells prepared:
Cantrips (at will): Booming Blade, Infestation
1st level (4 slots): Ray of Sickness, Absorb Elements
2nd level (3 slots): Blindness/Deafness, Hold Person, Suggestion
3rd level (3 slots): Haste, Dispel Magic, Counterspell
4th level (3 slots): Sickening Radiance, Wall of Fire
5th level (3 slots): Hold Monster, Cloudkill
6th level (2 slots): Summon Fiend, Sunbeam
7th level (2 slots): Crown of Stars, Symbol
8th level (1 slot): Maddening Darkness 
9th level (1 slot): Meteor Swarm

Action Name. Conquest attacks twice with Faith's Scimitar, Anti-Blade or uses spellcasting once.

Faith's Scimitar. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. Hit: 25 (6d6 + 4) slashing damage plus 55 (10d10) radiant damage. Conquest also regains half of the damage done as hitpoints.

 Anti-Blade. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. Hit: 25 (6d6 + 4) slashing damage plus 55 (10d10) necrotic damage. On a hit, the target can choose to make a DC 18 Constitution Saving Throw. If the target succeeds, it takes 44 (8d10) bludgeoning damage. If the target fails the Saving Throw, an sphere of magical darkness with a diameter of 20 feet is created with the target at its centre.

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

Conquested Mind (3/Day). If Conquest is targeted by and attack, it can choose to let the attacker make a DC 18 Wisdom Saving Throw. If the attacker fails, it must make its attack at one of its allies. It will be charmed until the end of Conquests next turn and has to attack its allies until then.