Find and kill Bonesunder in or near the canyon northwest of the Azure Dragonshrine. Bring any evidence you find back to Aurastrasza at Wyrmrest Temple.
Such a strange seal to see among magnataur... it suggests an unusual amount of organization.
Magnataur rarely put aside their aggressive natures to cooperate with each other. I suspect they have their eyes set on raiding a large settlement.
I very much doubt that this horn is unique, I need you to prove me wrong. In the canyon northwest of the Azure Dragonshrine you will find Bonesunder. Go challenge him and bring me back any evidence you find that might confirm my concerns.