Horn of the Traitor

Horn of the Traitor is the broken-off right horn of Cliffwatcher Longhorn, which was originally taken from his corpse as proof that the player brought the traitor to justice, then given back to them as a reward.

Using the horn summons Spirit of Cliffwatcher Longhorn, a lvl 43 guardian with 3,432 HP for 30 seconds. He is not happy about the prospect of being enslaved inside his own horn for all eternity.

Source
Rewarded from Horn of the Traitor (quest).