Linken's Boomerang

Linken's Boomerang is an trinket especially valuable to paladins, shamans and druids, as it allows them to have a physical ranged attack.

Source
This item is a quest reward from It's Dangerous to Go Alone.

Trivia
Like Linken, the boomerang is from  series. The stun proc is a reference to the fact that the Boomerang was almost always used to stun enemies.