Archaon The Everchosen

Critical mastery. Archaon's weapon attacks critically succeed on a natural 18 and 19 as well as a 20. When Archaon critically succeeds on a weapon attack, he triples the extra damage instead of doubling it.

Immense Strength. Archaon can wield a greatsword with one hand without suffering any impairments.

Innate Spellcasting. Archaon's innate spellcasting ability is Charisma (spell save DC 27, +18 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: evard's black tentacles hunger of hadar

4/day each: cloudkill, cone of cold,scrying

3/day each: circle of death, harm, soul cage

2/day each: reality break, power word stun, meteor swarm

1/dayeach: blade of disaster, time stop

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

Magic Resistance. The Everchosen has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Everchosen weapon attacks are magical.

Mark of Chaos. Demons connot willingly attack Archaon and have disadvantage on attacks against him if forced to. Archaon is immune to the effects of Divine Intervension abilities. When a character who is Good alignment first sees Archaon, they have to make a DC27 Wisdom saving throw. On a failure, they becomed stunned 1d10 rounds.

Three-Eyed. Archaon has advantage on all perception checks and characters have disadvantage on stealth checks to hide from him. Magical darkness doesn't impede Archaon's darvision

The Slayer of Kings. Melee Weapon Attack: +24 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 10) slashing damage.  Hit: 16 (2d6) fire damage.  Hit: 16 (2d6) necrotic damage. 

Parry. Archaon adds 8 to his AC against one melee attack that would hit him. To do so, Archaon must see the attacker and be wielding a melee weapon.

Archaon can take 3 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. Archaon regains spent legendary actions at the start of its turn.

Attack. Archaon makes one attack.

Herald of Chaos. Archaon summons any number of demons of his choice whos combined CR altogether equals no more than 26. The demons summoned cannot be the same as those summoned with the last use of this action.

Lesser Magic. Archaon casts a spell of 5th level or lower.

Greater Magic (Costs 2). Archaon casts a spell of 6th level or higher.