Death Spider

A terrifying twist of the polymorph spell, it turns the appendage no larger than 5 ft of a creature into a spider on a failed Charisma save. If the appendage is being used to hold something, that thing is dropped. If the appendage is necessary for the creature's movement, the creature's speed is halved. If the appendage is used for an attack, such as a tail, or claw attack, that attack is impossible while the spell is held and the spider is alive.

The spider starts with 5 HP and moves at the start of the target’s turn. Each round, it consumes part of the limb, dealing 2d6 piercing damage and gaining 5 HP, growing larger. The spider has 12 AC and can be killed, transforming it back into the limb it once was. If the target creature dies, the spider detaches itself from the creature and becomes its own creature with its own initiative, obeying your commands. It still has an AC of 12, the HP it had before detaching, and it has a bite attack with +5 to hit and 2d6 damage. It disappears once the spell ends.

At higher levels: the spider deals 1d6 extra damage per round, and with its bite attack per level of spell slot used above 5th.