Cosmic Consciousness Gauthak

Anathematic Secrecy. Gauthak can't be targeted by any divination magic or perceived through magical scrying sensors. Additionally, Gauthak has the properties of a Spelljamming Helm (albeit he moves 161 times faster) and a Wildspace Orrery, but he is the only one who can benefit from the item's effects (both are immune to dispel magic, antimagic, wild magic, jammerscreams). Finally, Gauthak’s actual Strength score is 62 (+26), his actual Dexterity score is 32 (+12), and his actual Constitution score is 59 (+24).

Aura of Transmigration. Any hostile creature that starts its turn within 60 feet of Gauthak must make a DC 24 Intelligence saving throw. On a failed save, a creature suffers disadvantage on all Wisdom and Charisma saving throws until the start of Gauthak’s next turn. A creature that fails the saving throw by 5 or more has all its actions, bonus actions, reactions, legendary actions, and mythic actions that it took since last turn negated.

Freedom of Movement. Gauthak ignores difficult terrain, and magical effects can't reduce his speed or cause him to be restrained. He can spend 5 feet of movement to escape from nonmagical restraints or being grappled.

Innate Spellcasting. Gauthak's innate spellcasting ability is Charisma (spell save DC 24). He can innately cast the following spells, requiring no components:

At will: bless, detect magic, dispel evil and good, call lightning

3/day each: blight, cloudkill, dawn, dispel magic

2/day each: contagion, fire storm, reverse gravity, foresight

1/day each: divine word, earthquake, feeblemind, time stop, wish (no chance of strain or misinterpertation)

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

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

Magic Weapons. Gauthak's weapon attacks are magical.

Mind Reader. Gauthak knows the surface thoughts of each creature within 1000 feet that isn’t protected by a mind blank spell or similar magic, and once per turn can use an free action to attempt to read a creature’s thoughts as with the detect thoughts spell. 

Unyielding Essence. Gauthak is immune to any spell or effect that would alter his form, as well as those that would read his thoughts, determine if he is lying, or magically influence his thoughts or behavior. He also doesn’t require air, food, drink, or sleep.

Multiattack. Gauthak makes three attacks: two with his Cyclic Censer and one with his Punarjanmanic Punch. He then uses Corporeal Instability.

Cyclic Censer. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 28 (6d6 + 7) slashing damage plus 7 (2d6) force damage, and the target is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the target takes 10 (3d6) force damage at the start of each of its turns.

Punarjanmanic Punch. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 36 (8d6 + 8) bludgeoning damage plus 7 (2d6) cold, psychic, lightning, radiant, or thunder damage (Gauthak’s choice).

Corporeal Instability. Gauthak targets one creature it can see within 60 feet of it. The target must succeed on a DC 24 Constitution saving throw or take 22 (4d6 + 8) force damage and become destabilized for 1 minute.
A destabilized creature's form constantly melts, flows, writhes, and boils. The creature has disadvantage on attack rolls, its speed is reduced by half, and whenever it takes damage, its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The creature dies if its hit point maximum is reduced to 0. A creature that dies in this way is erased from the past, present and future, and can only be brought back by means of a Wish spell or similar magic.
The destabilized creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Aimless Wandering (2/Day). One creature within 100 miles of the essence that the essence is aware of must make a DC 24 Wisdom saving throw. A creature automatically succeeds if it is immune to being charmed. On a failed save, the target must target its allies with attacks and other damaging effects for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the essence's Discord and Woe for the next 24 hours.

Samsaric Sundering (1/Day). As a bonus action, each creature within 100 miles of the elder evil that is a spellcaster or a user of arcane magic (a artificer, sorcerer, warlock, wizard, or some other class the DM deems appropriate) must make a DC 23 Constitution saving throw. A target takes 35 (10d6) force damage on a failed save, or half as much damage on a successful one. On a failed save, a target also suffers one level of exhaustion.

Gauthak can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Gauthak regains spent legendary actions at the start of its turn.

Reticent Revolution. Gauthak targets itself or one creature that is currently grappled by it, destabilized by it, affected by its Aimless Wandering or Samsaric Sundering, or affected by one of its ongoing spells. Uncontrolled particles burst out from the target, dealing acid, cold, fire, lightning, or thunder damage (Gauthak's choice) to those caught in the blast. Each creature in a 10-foot-radius sphere centered on the target must make a DC 24 Constitution saving throw, taking 14 (4d6) of the chosen type on a failed save, or half as much damage on a successful one. Gauthak is immune to this damage.

Fateful Fling. One Large or smaller object held or creature grappled by the essence is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 18 Dexterity saving throw or take the same damage and be knocked prone.

Partikinesis (Costs 2 Actions). Gauthak casts an innate spell.