Swans figure into in many a Celtic tale. In one story, Aengus falls asleep and dreams of Caer, a beautiful faerie maiden whom he falls instantly in love with. Her power over him is so great that a terrible sickness befalls him. When Aengus discovers that Caer is the daughter of Ethal of the Tuatha d é Danann, he immediately asks her father for Caer's hand in marriage. Ethal, however, states that does not have the power to grant this wish because his daughter has taken the form of a swan. He then explains to Aengus that if he can recognize her among a flock, he may ask her to marry him. Aengus goes to the Lake of the Dragon's Mouth and recognizes Caer immediately. He calls her name, asks her hand, and the two eventually marry.