No Heart, No Soul. The World Brain has no care for the lives of anything but itself

The End of Days. Once every short rest, the World Brain can either destroy or dominate a 500 by 500 foot area. The save DC for any affected creatures for either effect is 25. It will be a Wisdom saving throw for domination and a Dexterity saving throw for destruction. on a failed save, the creature(s) must take either 40d12 (260) psychic damage, or be subject to the World Brain's will for 30 days.

Omniscient. The World Brain, through its seeping of knowledge, knows all languages

Spellcasting. The World Brain is a 18th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 23, +15 to hit with spell attacks). The World Brain has the following Wizard spells prepared:
Cantrips (at will): Mage Hand, Message, Minor Illusion
1st level (4 slots): Absorb Elements, Earth Tremor, Ray of Sickness

2nd level (3 slots): Blindness/Deafness, Crown of Madness, Earthbind

3rd level (3 slots): Fear, Lightning Bolt, Major Image, Sleet Storm

4th level (3 slots): Banishment, Black Tentacles, Phantasmal Killer

5th level (3 slots): Cloudkill, Danse Macabre, Synaptic Static

6th level (1 slot): Circle of Death, Soul Cage

7th level (1 slot): Power Word Pain, Sequester

8th level (1 slot): Maddening Darkness, Mind Blank

9th level: (1 slot): Psychic Scream

Multiattack. The World Brain gets two attacks per turn, one with its tentacles and one spell.

Tentacles. Melee Weapon Attack: +8 to hit, reach 15 ft., 1 target. Hit: 24 (5d8 + 1) bludgeoning and 7 (2d6) poison damage. If hit, the creature must make a DC 20 Dexterity saving throw or be grappled. The World Brain can grapple up to two creatures at one time.

Mind Transfer. The World Brain can transfer some knowledge from a creature it has grappled, allowing it to learn a feature of it (class, racial, etc.) and use it on its next turn.

The World Brain can take a Legendary Action at the end of another creature's turn. It has 4 actions to use, which are restored when it returns to the World Brain in initiative order..

Cast A Spell (Costs 1 Action). The World Brain can cast any one of the spells it knows.

Tentacle Swarm (Costs 2 Actions). The World Brain flails its tentacles about in a frenzy. All creatures within 20 feet of it must make a DC 17 Dexterity saving throw. The targets must take 54 (12d8) bludgeoning and 18 (5d6) poison damage on a failed save, or half on a success.

Intellect Devour (Costs 4 Actions). The World Brain targets a creature within 50 feet. The creature must make a DC 18 Intelligence saving throw or have their intelligence score dropped to 6. The World Brain regains hitpoints equal to 3 times the number subtracted from the creature's Intelligence score.