Ancient Elder Brain

Creature Sense. The elder brain 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.

Innate Spellcasting (Psionics). The elder brain's innate spellcasting ability is Intelligence (spell save DC 20). The elder brain can innately cast the following spells, without providing material components:

At will: detect thoughts, levitate, shield

3/day each: dominate monster, telekinesis

1/day each: plane shift, psychic scream

Legendary Resistance (3/Day). If the elder brain fails a saving throw, it can choose to succeed instead.

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

Telepathic Hub. The elder brain can use its telepathy to initiate and maintain telepathic conversations with up to ten creatures at a time. The elder brain can let those creatures telepathically hear each other while connected in this way.

Multiattack. The elder brain makes two tentacle attacks.

Tentacle. Melee Weapon Attack: +12 to hit, reach 40 ft., one creature. Hit: 28 (5d8 + 6) bludgeoning damage. If the target is Huge or smaller, it is grappled (escape DC 16) and and takes 15 (2d8 + 6) psychic damage at the start of each of its turns until the grapple ends. The elder brain can have up to four targets grappled at a time.

Mind Blast (Recharge 5-6). The elder brain magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 20 Intelligence saving throw or take 39 (6d10 + 6) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Psychic Link. The elder brain targets one incapacitated creature it can perceive with its Creature Sense trait and establises a psychic link with that creature. Until the psychic link ends, the elder brain 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 elder brain can terminate the link at any time (no action required). The target can use an action on its turn to attempt to break the psychic link, doing so with a successful DC 20 Charisma saving throw. On a successful save, the target takes 14 (4d6) psychic damage. The psychic link also ends if the target and the elder brain are more than 5 miles apart, with no consequences to the target. The elder brain can form psychic links with up to ten creatures at a time.

Sense Thoughts. The elder brain targets a creature with which it has a psychic link. The elder brain gains insight into the target's reasoning, its emotional state, and thoughts that loom large in its mind (including things the target worries about, loves, or hates}. The elder brain can also make a Charisma (Deception) check with advantage to deceive the target's mind into thinking it believes one idea or feels a particular emotion. The target contests this attempt with a Wisdom (Insight) check. If the elder brain succeeds, the mind believes the deception for 1 hour or until evidence of the lie is presented to the target.

The elder brain 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 elder brain regains spent legendary actions at the start of its turn.

Tentacle. The elder brain makes a tentacle attack.

Break Concentration. The elder brain targets a creature within 120 feet of it with which it has a psychic link. The elder brain breaks the creature's concentration on a spell it has cast. The creature also takes 1d6 psychic damage per level of the spell.

Psychic Pulse. The elder brain targets a creature within 120 feet of it with which it has a psychic link. Enemies of the elder brain within 10 feet of that creature take 14 (4d6) psychic damage.

Sever Psychic Link. The elder brain targets a creature within 120 feet of it with which it has a psychic link. The elder brain 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.

Cast Spell (Costs 3 Actions). The elder brain casts a spell.