Avatar of Azathoth, Monolithic God of Chaos and Madness

Madness Aura: Any creature that can see Azathoth or is within 300 feet of it and is aware of it must succeed on a DC 25 Wisdom saving throw or become frightened for 1 minute and roll on the Short-term Madness table. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Immortal: Azathoth cannot permanently die. When Azathoth is destroyed he reforms himself in the Far Realms after 1d100 years.

Spellcasting. Azathoth is a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 25, +17 to hit with spell attacks). Azathoth has following spells prepared:
at will (as 8th level): disintegrate, finger of death, power word stun, slow
3 times per day (as 9th level): meteor swarm, summon greater demon, true polymorph, imprisonment, mass suggestion, plane shift, dark star
1 per day: time stop, wish, power word kill

Multiattack: Azathoth makes five attacks divided freely between its available tentacles, claws, and mouths (bite).

Tentacle: Melee Weapon Attack: +24 to hit, reach 60 ft., one target. Hit: 30 (4d10 + 10) bludgeoning damage and the target is grappled (escape DC 24)and pulled 15ft. toward Azathoth.

Claw: Melee Weapon Attack: +24 to hit, reach 30 ft., one target. Hit: 20 (3d8 + 10) slashing damage.

Bite: Melee Weapon Attack: +24 to hit, reach15 ft., one target. Hit: 40 (6d10 + 10) piercing damage.

Swallow: Azathoth makes one bite attack against a Huge or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside of Azathoth and it takes 56 (16d6) necrotic damage at the start of each of Azathoth's turns. If Azathoth takes 75 damage or more on a single turn from a creature inside it, Azathoth must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Azathoth. If Azathoth dies, a swallowed creature is sucked through a gate to a random location on the Far realms as Azathoths form returns to its place of origin. The gate then closes. The gate is one-way only and can’t be reopened.

Amorphous: Azathoth can use different appendages to attack and can change what limbs are available as a bonus action, using any combination of up to five tentacles, claws, or mouths.

Azathoth can take 5 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. Azathoth regains spent legendary actions at the start of its turn.

Cataclysm (3 Actions). Azathoth unleashes a devastating blast of chaotic energy in a 120-ft. radius sphere centered on itself. All creatures within the area must make a DC 25 Dexterity saving throw, taking 100 (20d10) force damage on a failed save, or half as much on a successful one. Additionally, the area becomes a twisted, chaotic landscape, and all structures and objects are destroyed.

Move (1 Action). Azathoth moves up to its speed without provoking opportunity attacks.

Attack (1 Action). Azathoth makes one tentacle attack.

Cast a Spell (2 Actions). Azathoth cast a spell it knows.

Reality Collapse: Azathoth unleashes a devastating wave of reality-bending energy, warping the very fabric of existence.
All creatures within a 1-mile radius of Azathoth must make a DC 25 Intelligence saving throw. On a failed save, a creature takes 10d10 psychic damage and is stunned for 1 minute. On a successful save, a creature takes half as much damage and isn't stunned. 
In addition, any structures or objects, in the radius are instantly disintegrated as the fabric of reality is torn apart. This ability can only be used once per 1d100 years, and automatically triggers when Azathoth's Health is equal to or less than 100.
After using this ability, Azathoth regains 50d12 + 400 hit points.

Reality Warp (3 Actions). Azathoth manipulates reality itself, warping the very fabric of existence to its will. The effects of this ability are unpredictable and can manifest in a variety of ways, such as altering the terrain, summoning strange creatures, or distorting the laws of physics. Roll a d6 to determine the effect.

1). Gravity Distortion: The gravity within the area becomes erratic, causing creatures to float or be pulled in different directions. All creatures within the area must make a DC 25 Strength or Dexterity saving throw (their choice) or be restrained by the strange gravitational forces for 1 minute. At the start of each of their turns, they can make another saving throw to break free.

2). Reality Shift: The fundamental laws of physics within the area are altered, causing strange and unpredictable effects. Roll a d10 to determine the effect (1-2: all creatures within the area become weightless, 3-4: all creatures within the area become invisible, 5-6: all spells cast within the area have their effects reversed, 7-8: all objects not being worn or carried within the area are teleported to a random location within 100 miles, 9-10: roll twice on this table, ignoring any results of 9-10, or duplicates.

3). Terrain Warp: The landscape within the area begins to shift and change in strange ways. The ground may become unstable, walls may appear or disappear, or strange obstacles may hinder movement. All creatures within the area must make a DC 25 Dexterity saving throw or fall prone and take 20d6 bludgeoning damage on a failure, or half damage and not be knocked prone on a success.

4). Reality Tear: A rift in reality opens up, causing strange and dangerous creatures to spill forth. Roll a d10 to determine what creatures emerge (1-3: a Cosmic Horror (Spelljammer), 4-6: a Balor, 7-9: a Leviathan (Monsters of the Multiverse), 10: Iggwilv the Witch Queen (Wild Beyond the Witchlight) ). The creatures remain for 1 minute before disappearing back into the rift. The creatures are hostile to all creatures in the area.

5). Time Dilation: Time slows or speeds up within the area. Roll a d6 to determine the effect (1-3: time slows to half speed, all creature in range of Azathoth's lair must make a DC 25 wisdom saving throw or suffer the effects of the Slow spell. 4-5: time speeds up to double speed, every creature within range of Azathoth's lair gains the benefits of the Haste spell, and are not stunned when the spell wears off. 6: Azathoth gains the effect of the Time Stop spell as if Azathoth were the caster).

6). Spatial Shift: All creatures other than Azathoth within the range of Azathoth's lair must make a DC 25 Intelligence saving throw or take 20d6 psychic damage and become stunned for one round on a failure, then are teleported to a random location within 300 feet regardless of success or failure of the save.