Source: The Book of Many Things

Weapon (any ammunition), uncommon

This leech has been dried and imbued with a mote of animating magic. If you hit a creature with a ranged attack roll using this ammunition, the leech springs to life and sinks its teeth into the target, dealing 1d4 piercing damage at the start of each of the target’s turns. If the leech deals at least 10 damage or the target dies, the leech falls off. A creature, including the target, can use its action to detach the leech. Once a leech is no longer attached to its target, the leech dies and is no longer magical.