Upon arrival at the Keep, walk forward a bit and a soldier will instruct you to lure the Golden Deer to their area. You don't need to fight or kill it - once the deer reaches that spot it will be captured and the quest will be completed. At the same time, until you leave the Keep, take the opportunity to explore the rest of the area at your leisure without the threat of the Deer. If your party did manage to fight and kill the Deer, on the other hand, turn in the Golden Horn it drops to complete the quest instead - at the cost of a lower reward.