Death Spider

You touch a creature, and it must immediately make a Wisdom saving throw. On a failed save, one of its limbs turns into a mindless, hairy spider. On the target's next turn, the spider twists around and starts to devour its host. The target then takes 1d6 piercing damage at the start of each of its turns.

Dispel magic can only halt the spell and save the limb if it is cast before the end of your next turn after death spider is cast. Otherwise, the spider limb must be hacked off (15 AC, 40 hp, vulnerable to slashing damage), which kills the spider. If the spider is not stopped, it will continue to inflict damage on its host, consuming it when it reaches 0 hp, and then growing into a giant spider over the next 24 hours.

“This surely ranks as one of the most disturbing spells ever devised. It was created by the Drow as a way to combine execution with entertainment.” —Elminster

“Elminster is a wasteful gossip with no regard for efficiency. This spell replaces an enemy with a servant, or an incompetent servant with a better one. I only wish it were quicker about it.” —Acererak