Bouncing Ring

A small steel ring one would wear on a finger. The outside is etched with an erratic array of straight geometrical lines. Despite seemingly being made of metal, the ring is incredibly bouncy as if made of rubber.

As a bonus action, you may throw this item at a hard surface. When thrown in this way in a confined space, it begins ricocheting violently, bouncing with enough force to injure those who it impacts. 
The bouncing ring fills a 20' cube, and moves 10' in a random direction on initiative 0 each turn. A creature takes 2d4 bludgeoning damage when it enters the ring's area for the first time on a turn, or starts its turn there. This effect lasts 1 minute, and the ring is rendered useless afterwards.