Cranium Wererat

Shapechanger. The cranium wererat can use its action to polymorph into a cranium rat-humanoid hybrid or into a giant cranium rat, or back into its true form, which is humanoid. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed.

Keen Smell. The wererat has advantage on Wisdom (Perception) checks that rely on smell.

Illumination. The cranium wererat can use a bonus action to shed dim light from its brain in a 10-foot radius or extinguish the light.

Innate Spellcasting (Psionics). The cranium wererat's spellcasting ability is Intelligence (spell save DC 14) and can innately cast the following spells, requiring no components:

At will: comprehend languages, dominate monster (target must be a cranium rat or swarm of cranium rats)

 1/day each: levitate, detect thoughts

Multiattack (Humanoid or Hybrid Form Only). The wererat makes two melee attacks, only one of which can be a bite.

Bite (Rat or Hybrid Form Only). Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 3) piercing damage. If the target is a humanoid, it must succeed on a DC 14 Constitution saving throw or be cursed with wererat lycanthropy.

Tentacles (Humanoid or Hybrid Form Only). Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 3) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 14) and must succeed on a DC 14 Intelligence saving throw or be stunned until this grapple ends.

Shortsword.  Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 3) piercingdamage.

When dominating a Swarm of Cranium Rats, the Cranium Wererat has 2 Legendary actions it can use per round; if the Swarm has less than half its hit points remaining, the Cranium Wererat only has 1 Legendary action it can use per round and cannot use its Psionic Blast. The Cranium Wererat can only use one legendary action option at a time and only at the end of another creature's turn. All spent legendary actions are regained at the beginning of this creature's turn.

Move. The Cranium Wererat moves up to its speed without provoking attacks of opportunity.

Melee attack. The cranium wererat can make one melee attack based on its current form. 

Psionic Blast. Ranged Spell Attack:  +7 to hit, reach 30 ft., one target. Hit: 11 (2d6 + 3) psychic damage.