Waltor of Pal'ea at the Coast of Echoes in the Borean Tundra wants you to use the Horn of the Ancient Mariner to summon Orabus the Helmsman. Once summoned, slay Orabus the Helmsman.
Take the horn and travel west to the Warsong Jetty. Make your way to the end of the jetty and blow into the horn. The sound should rouse Orabus.
Slay the foul beast and return to me!