5. The Faceless Men
Getting into theory territory, although very much one of the
more believable ones, it is suggested the Euron is actually the one responsible for his brother Balon's mysterious death. In the books, Balon falls from a high bridge connecting two of Pyke's rocky spires, and Euron - who hasn't been seen for a couple of years at this point - suddenly turns up the next day to stake his claim for the Iron Throne. The Ghost of High Heart, whose other visions that she tells to Arya and the Brotherhood Without Banners come true, says: "I dreamt of a man without a face, waiting on a bridge that swayed and swung. On his shoulder perched a drowned crow with seaweed hanging from his wings." That certainly points to the Faceless Men of Braavos, and Euron 'The Crow's Eye' Greyjoy. Given his travels he'd probably have knowledge of the assassins, and also a clear motive, and this seems like one theory that is most likely true, and we might get to see it play out in season 6.