Azathoth

Outer God's Wrath (Recharge after a Short or Long Rest). If Azathoth would be reduced to 0 hit points, the skull that control the entire body breaks and his current hit points total instead resets to 200 hit points. He regains any expended uses of Legendary Resistance, but it can no longer use his actions Arcana Burst and Spellcasting, it can no longer use his bonus action ectoplasm summoning, it can no longer use his legendary actions Spells and Awaken my Servants! and his level of Intelligence, Wisdom and Charisma drop to 6 (-2), but additionally, Azathoth can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 75.000 XP (180.000 XP total) for defeating Azathoth after his Outer God's Wrath activates.

Legendary Resistance (3/Day). If Azathoth fails a saving throw, he can choose to succeed instead.

Multiple Nature. Azathoth is also considered an Undead.

Magic Resistance.  Azathoth has advantage on saving throws against spell and other magical effects.

Spider Climb. Azathoth can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Multiattack. Azathoth makes three Arcana Burst attack. If Azathoth already used his Outer's God's Wrath, it makes two Bite attack and three Pseudopod attack instead.

Pseudopod. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage. 

Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 8) piercing damage plus 22 (4d10) necrotic damage. 

Arcana Burst. Ranged or Melee Magical Attack: +13 to hit, reach 5 ft. or range 120 ft., one target. Hit: 32 (5d10 + 5) necrotic damage.

Spellcasting. Azathoth casts one of the following spells, using Intelligence as spellcasting ability (+13 to hit with spells, spell save DC 21):

At will: detect magic, dancing lights, mage hand, chill touch

3/day: spirit shroud (TCoE), teleport, melf's acid arrow  bestow curse, invisibility

2/day:  fireball, lightning bolt, cone of cold

1/day: finger of death, harm, prismatic spray

Ectoplasm Summoning. If azathoth is near a corpse that died less than an hour ago, Azathoth magically turns that corpse in a Burned (if the corpse has been killed by fire damage), in a Drowned (if the corpse has been killed by cold damage or if he died by drowning), in an Asphyxiated (if the corpse has been killed by lightning damage or if he died by suffocation) or in a Buried (if the corpse has been killed by acid damage or if he died buried). The summoned ectoplasm start its turn after that of Azathoth and follows his orders (no action are required). The ectoplasm stays until it dies or until Azathoth death.

Azathoth 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. The monster regains spent legendary actions at the start of its turn.

Attack. Azathoth makes one Arcana Burst, Pseudopod or Bite attack.

Spells. Azathoth cast a spell.

Movement. Azathoth moves.

Awaken my Servants! (Costs 3 Actions) (1/Day). Each Burned, Drowned, Asphyxiated and Buried died during the last hour, within 120 ft. from Azathoth, is resurrected with all its hit points.

If Azathoth's Outer God's Wrath is active, it can use the options below as legendary actions for 1 hour after using.

Mephitic Scream (Costs 2 Actions). Azathoth emits in a radius of 60 ft. from him a black miasma. The miasma spreads around corners and remains for one hour. A creature with darkvision can’t see through this darkness, and nonmagical light can’t illuminate it. Every creature that needs to breath that stay in that area for one consecutive minute dies for suffocation. Each Burned, Drowned, Asphyxiated and Buried that stay inside the miasma obtain resistance to nonmagical melee attacks and they can make an extra pseudopod attack per turn. A moderate wind (at least 9 mph) scatters the wind in 4 round. A strong wind (at least 18 mph) scatters the wind in 1 round. If Azathoth uses this action twice, the first one scatters.

Sacrifice yourself my paws! (Costs 3 Actions). Each Burned, Drowned, Asphyxiated and Buried in a radius of 120 ft. from Azathoth blows in an explosion of 9 meter radius. Each creature in that radius must succeded on a DC 21 Dexterity saving throws for each explosion in which is involved or take 27 (6d8) fire damage if a Burned exploded, cold damage if a Drowned exploded, lightning damage if an Asphyxiated exploded or acid damage if a Buried exploded. The exploded ectoplasm dies and cannot be resurrected unless a wish spell is used.