Asteraoth, The Lord of Madness

Insanis' Avatar. Any creature hostile to Asteraoth that starts its turn within 20 feet of him or a creature subjected to his telepathic communication must make a DC 24 Intelligence saving throw, unless Asteraoth is incapacitated. On a failed save, the creature starts its mind. Roll an effect on the short-termed madness table (DMG pg. 259). If the creature fails another saving throw while still affected by that effect, madness starts to root itself into its mind. Roll on the long-termed madness table (DMG pg. 260) to determine the effect to replace the previous one. If the creature fails another saving throw while still affected by this effect, madness is fully incorporated into its personality. Roll on the indefinite madness table (DMG pg. 260) to determine which flaw the creature develops to override its long-term madness. If that creature fails another saving throw while still affected by the new flaw, its mind breaks apart. Its Intelligence becomes 0 and the target is stunned until it regains at least one Intelligence point.

Legendary Resistance (2/day). If Asteraoth fails a saving throw, he can choose to succeed instead.

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

Magic Weapons. Asteraoth's weapon attacks are magical.

Innate Spellcasting. Asteraoth's spellcasting ability is Intelligence (spell save DC 24). Asteraoth can innately cast the following spells, requiring no material components:

At will: mirror image, misty step, disguise self, mislead, phantasmal force

3/day each: fear, greater invisibility, hallucinatory terrain, phantasmal killer, seeming

2/day each: mental prison, mirage arcane, simulacrum

1/day: weird

Multiattack. Asteraoth makes three attacks: one with its bite, one with its claw, and one with its tail.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 11 (1d6 + 8) piercing damage.

Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 12 (1d8 + 8) slashing damage.

Tail. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 13 (1d10 + 8) bludgeoning damage.

Asteraoth, Lord of Madness, can take 2 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. Asteraoth regains spent legendary actions at the start of his turn.

Attack. Asteraoth makes a bite, claw or tail attack.

Teleport (Costs 2 Actions). Asteraoth magically teleports, along with any equipment it is currently wearing or carrying, or anything he is holding or creature he is grappling, up to 120 feet to an unoccupied space he can see.

Cast a Spell (Costs 2 Actions). Asteraoth casts a spell from the list of spell available to him that has a casting time of 1 action.