Assistant Brain

Adept Spellcaster. The assistant brain can use a bonus action to cast all spells

Spellcasting. The assistant brain is a 1st-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 16, +4 to hit with spell attacks). The assistant brain has following [class name] spells prepared:
Cantrips (at will): Mage Hand, Minor Illusion (To make tentacles only) 

Tentacle Runways A creature can move on the tentacles of the assistant brain as if it where difficult terrain with a successful DC 12 Dexterity (Acrobatics) check. If the assistant brain makes the Tentacle Web reaction, a character can make an attack roll against the tentacle as an action or a bonus action (Tentacle AC 10). On a success,  that tentacle is chopped off, and cannot make another attack until the assistant brain uses it's bonus action to regenerate it's tentacles. A creature can attack both tentacles with their action and bonus action.

Multi Attack The assistant brain can make two tentacle attacks

Tentacle Slam. Melee Weapon Attack: +2 to hit, Anywhere in the arena., 1 target. Hit: 3 (1d4 + 1) bludgeoning damage. 

Mental Screech (Recharge 5–6). The assistant brain emits a psychic screech telepathically. Each creature within 20 feet must succeed on a DC 12 Intelligence saving throw or take 5 (1d4 + 3) psychic damage.

The assistant brain can regenerate all lost tentacles as a bonus action.

Tentacle Web. When a creature tries to run on the tentacles of the assistant brain, the brain may make 2 tentacle slam attacks to that creature.