Detect Sentience.

The brain can sense the presence and location of any creature within 300 feet of it that has an Intelligence of 3 or higher, regardless of interposing barriers, unless the creature is protected by a mind blank spell.

Innate Spellcasting (Psionics).

The brain's innate spellcasting ability is Intelligence (spell save DC 14, +6 to hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: chill touch (see "Actions" below), detect thoughts, mage hand, zone of truth
3/day each: charm person, hold person
1/day each: compulsion, hold monster, sleep (3rd-level version), Tasha's hideous laughter

Magic Resistance.

The brain has advantage on saving throws against spells and other magic effects.

Unusual Nature.

The brain doesn't require air, food, drink, or sleep.

Actions

Chill Touch (Cantrip).

Ranged Spell Attack: +6 to hit, range 120 ft., one creature. Hit: 13 (3d8) necrotic damage, and the target can't regain hit points until the start of the brain's next turn. If the target is undead, it also has disadvantage on attack rolls against the brain until the end of the brain's next turn.

Mind Blast (Recharge 5–6).

The brain magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 14 Intelligence saving throw or take 17 (3d8 + 4) 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.

Brain in a Jar