A Hallowed Horn - Skellige
Location: North of Arinbjorn
Quest Giver: Holmstein Notice Board/Stumbling into Bandits
Other Requirements: N/A
You can pick up this quest from the Holmstein Notice Board or from randomly stumbling into the camp of bandits out in your travels. The location of the bandit camp is shown in my screen shot above, it's to the north of Arinbjorn and south of the lighthouse you have to visit for the Contract: The Phantom of Eldberg Quest.
For this quest basically all you have to do is kill the bandits at the camp, loot the horn and the journal then return to Mathios to turn in the quest. If you didn't get this quest from the Holmstein Notice Board then you'll need to read the journal to know who to turn it in to.