Robe of Serpents

From Storm King's Thunder, page 236.

Description
Wondrous item, uncommon (requires attunement).

A robe of serpents is a stylish silk garment that is popular among wealthy nobles and retired assassins. The robe is emblazoned with 1d4+3 stylized serpents, all brightly colored.

As a bonus action on your turn, you can transform one of the robe's serpents into a giant poisonous snake. The snake instantly falls from the robe, slithers into an unoccupied space next to you, and acts on your initiative count. The snake can tell friendly creatures from hostile ones and attacks the latter. The snake disappears in a harmless puff of smoke after 1 hour, when it drops to 0 hit points, or when you dismiss it (no action required). Once detached, a snake can't return to the robe. When all of the robe's snakes have detached, the robe becomes a nonmagical garment.