Magic Resistance. The Great Old One has an advantage on saving throws against spells and other magical effects.

Pure Chaos:  At 1000 hp The Great Old One regains 1/3 of his hp, grows to be 40 meters tall with looks like a black and silver liquid behemoth. He gains an advantage on all attacks at this time.

Reality Break. The Great Old One has the ability to break reality and bend it to his will. 

Spellcasting. The Great Old One is a 19th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). It has the following paladin spells prepared:

1st level (4 slots): command, compelled duel, arms of hadar, entangle

2nd level (3 slots): hold person, crown of madness, phantasmal force, tasha's mind whip

3rd level (3 slots): dispel magic, hunger of hadar

4th level (3 slots): black tentacles, shadow of moil

5th level (2 slots): telekinesis

6th level (2 slots): blade barrier

7th level (2 slots): teleport

8th level (2 slots): antimagic field

9th level (2 slots): summon shadowspawn

Multiattack. The Great Old One makes four Fist attacks or one Chaos Blade attack. You can use two Tendrils with the Fist attacks or Chaos Blade attacks.

Fist. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 44 (5d8 + 4) bludgeoning damage.

Tendril. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 36 (4d8 + 4) bludgeoning damage.

Chaos Blade. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 93 (11d8 + 5) slashing damage, or 115 (11d10 + 5) slashing damage if used with two hands, plus 32 (4d8) necrotic damage.

Chaos Orb (1/Day). The Great Old One hurls a magical ball of Chaos that explodes at a point it can see within 240 feet of it. Each creature in a 20-foot-radius sphere centered on that point must make a DC 18 Dexterity saving throw. The sphere spreads around corners. A creature takes 90 (18d10) fire damage and 35 (10d6) necrotic damage on a failed save, or half as much damage on a successful one.