You can possess a creature of meduim or small size with a CR rating equal to half your current level (rounded up). If the creature is dead you can re-animate, but any missing body parts cannot be grown back, if the creature is living it will die after wearing the cloak for 5 minutes plus the intelligence modifier of the creature. Whilst trying to possess a creature, any damage taken will fail the attempt, and both parties receive the "grappled" condition. 

Eg: A creature with +2 intelligence will take 7 minutes to die, but a creature with -4 intelligence will take 1 minute to die. 

On a successful possession, you gain the ability to speak any known languages and receive temporary hitpoints equal to the creature's remaining hp or half their maximum hp (whichever is highest). These hitpoints persist over long rests. 

Charisma score increases by 2.

Constitution score decreases by 1.

Your size is small by default but depends on what creature is possessed. 

As a cloak, you have resistance against blugeoning damage as any attacks will struggle to piece the fabric. Whilst grappling a creature, they are silenced.