S5·E3 The Death Song of Uther Pendragon

When a stranger gives Arthur the power to summon the dead, the king finds himself torn between head and heart. Unable to resist temptation, he seizes the chance to speak to the person he misses most – his father, Uther Pendragon. But the spirit world is a dark and dangerous place, for little does Arthur realize that his decision comes at a terrible price. Against all odds, Merlin must put right Arthur's mistake before Camelot, and everything they have built, is destroyed forever.