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.

Trivia
The stun proc is a reference to the fact that the Boomerang from the Legend of Zelda series was almost always used to stun enemies.

"Linken" is a parody of Link, from the Legend of Zelda series, as the boomerang from the Legend of Zelda series can loop around and kill minor creatures, and stun higher ones.