Giants

If the characters claimed Skyreach Castle in Hoard of the Dragon Queen, they might well hear from a frost giant diplomat who shows up to reclaim it as property of the giants who built it. A frost giant named Harshnag lives on Mount Sar north of Waterdeep, and he is sometimes called upon when the Sword Coast faces dire threats. The player characters could fly the citadel to him or—in a more dramatic interlude—he might come to a council meeting and demand the citadel’s return on behalf of his kin so that he might rally them against their ancient foes.