Azeroth

Creature Sense. The ulitharid is aware of the presence of creatures within 2 miles of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each creature’s Intelligence score, but can’t sense anything else about it. A creature protected by a mind blank spell, a nondetection spell, or similar magic can’t be perceived in this manner.

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

Legendary Resistance (Psychoelectric Awakening only) (3/Day). If the ulitharid fails a saving throw, it can choose to succeed instead.

Psionic Hub. If an elder brain establishes a psychic link with the ulitharid, the elder brain can form a psychic link with any other creature the ulitharid can detect using its Creature Sense. Any such link ends if the creature falls outside the telepathy ranges of both the ulitharid and the elder brain. The ulitharid can maintain its psychic link with the elder brain regardless of the distance between them, so long as they are both on the same plane of existence. If the ulitharid is more than 5 miles away from the elder brain, it can end the psychic link at any time (no action required).

Psychoelectric Awakening (Recharges after a Short or Long Rest). If the ulitharid would be reduced to 0 hit points, its current hit point total instead resets to 100 hit points, it recharges its Mind Blast, it regains its uses of psionics, it regains all of its spell slots, and it regains any expended uses of Legendary Resistance, and it gains advantage on strength checks. Additionally, the ulitharid can now use the options in the “Mythic Actions” section for 1 hour.

Tentacles. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. Hit: 27 (4d10 + 5) psychic damage. If the target is Large or smaller, it is grappled (escape DC 15) and must succeed on a DC 18 Intelligence saving throw or be stunned until this grapple ends.

Mind Blast (Recharge 5–6). The ulitharid magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 18 Intelligence saving throw or take 31 (4d12 + 5) psychic damage and be stunned for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Spellcasting (Electropsionics). The ulitharid casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

At will: detect thoughts, levitate, mind spike, witch bolt, absorb elements (lightning only), magnify gravity, zephyr strike

5/day each: lightning arrow, telekinesis, counterspell, call lightning, pulse wave

3/day each: steel wind strike, arcane hand, gravity sinkhole,

1/day each: dominate monster, feeblemind, mass suggestion, plane shift (self only), project image, scrying,

Spellcasting. The mind flayer arcanist is a 13th-level spellcaster. Its spellcasting ability is Intelligence (save DC 18, +10 to hit with spell attacks). The mind flayer arcanist has the following wizard spells prepared:

Cantrips (at will): mage hand, shocking grasp, mind sliver, vicious mockery, lightning lure

1st level (5 slots): detect magic, disguise self, shield, dissonant whispers, jim's magic missile

2nd level (4 slots): mirror image, tasha's mind whip, shadow blade

3rd level (4 slots): clairvoyance, lightning bolt, sending

4th level (4 slots): confusion, hallucinatory terrain, raolothim's psychic lance

5th level (3 slots): synaptic static, far step

6th level (2 slots): gravity fissure, chain lightning 

7th level (2 slots): reverse gravity, power word pain

Psychic Link. The ulitharid targets one creature it senses with its Creature Sense trait and forces that creature to make a Charisma saving throw (DC 18). On failure, the ulitharid establishes a psychic link with the target. Until the link ends, the ulitharid can perceive everything the target senses. The target becomes aware that something is linked to its mind once it is no longer incapacitated, and the ulitharid can terminate the link at any time (no action required). The target can use an action on its turn to attempt to break the link, doing so with a successful DC 18 Charisma saving throw. The link also ends if the target and the ulitharid are more than 2 miles apart. The ulitharid can form psychic links with up to five creatures at a time.

Magic Reversal. When the ulitharid uses its Legendary Resistance to negate a damaging spell effect, it casts witch bolt as a reaction.

Telepathic Parry (Psychoelectric Awakening Only). When the ulitharid is hit by a melee attack, it can half the damage as a reaction. It then makes one claw attack as part of the same reaction.

The ulitharid can take 3 legendary actions, choosing from the options below. It can take only one legendary action at a time and only at the end of another creature’s turn. The ulitharid regains spent legendary actions at the start of its turn.

Break Concentration. The ulitharid targets one creature within 120 feet of it with which it has a psychic link. The ulitharid breaks the creature’s concentration on a spell it has cast. The creature also takes 2 (1d4) psychic damage per level of the spell.

Psychic Pulse. The ulitharid targets one creature within 120 feet of it with which it has a psychic link. The target and enemies of the ulitharid within 30 feet of target take 10 (3d6) psychic damage.

Sever Psychic Link. The ulitharid targets one creature within 120 feet of it with which it has a psychic link. The ulitharid ends the link, causing the creature to have disadvantage on all ability checks, attack rolls, and saving throws until the end of the creature’s next turn.

Tentacle (Costs 2 Actions). The ulitharid makes one Tentacle attack.

Spellcasting. The ulitharid casts a spell from either spell list

If the ulitharid’s Chromatic Awakening trait has activated in the last hour, it can use the options below as legendary or normal actions.

Normal Actions

Multiattack. The ulitharid makes three claw attacks or three punch attacks. It then attempts to grapple one target

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 4) slashing damage plus 10 (2d10) lightning damage

Punch. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d10 + 3) bludgeoning damage and the target must succeed on a Strength saving throw (DC 15) or be knocked prone.

Legendary Actions

Claw. The ulitharid makes one claw attack.

Spell (lesser). The ulitharid casts one spell of 4th level or lower.

Recharge mind blast. The ulitharid recharges its mind blast. Its mind blast automatically recharges for the ulitharid's next three turn.

Psychoelectric Spear. Ranged Spell Attack: +10 to hit, range 120 ft., one target. Hit: 16 (2d10 + 6) psychic damage and 10 (2d10) lightning damage.

Buster. With a target grappled by the ulitharid, the ulitharid leaps into the air. Roll 5d10 and round to the nearest ten. That is how high the ulitharid jumps, and the target takes adequate fall damage.

Mind Blast (cost 2 actions). The ulitharid uses its mind blast.

Spell (greater) (costs 2 actions). The ulitharid casts one spell of 5th level or greater.

Psychoelectric Blast (costs 2 actions). All creatures within 60 feet of the ulitharid must make a DC 18 intelligence saving throw. On failure, the creature takes 15 (3d10) psychic damage and 15 (3d10) lightning damage. On success, the target takes no psychic damage and half lightning damage.