On the voyage home, Theseus abandons Ariadne on the island of Naxos, for reasons ancient sources give variously as forgetfulness, a directive from a god who wanted her for himself, since Dionysus later marries her there, or simple faithlessness on Theseus's part; the episode is told with considerably more sympathy for Ariadne in some retellings than others. Preoccupied with these events, Theseus forgets to carry out an arrangement made with his father before departing Athens: if the mission succeeded, the ship was to raise white sails on its return instead of the black sails it had sailed out under as a mark of mourning.
Watching from a clifftop for his son's ship, Aegeus sees the black sails still flying and, believing Theseus dead, throws himself into the sea, which is said to take its name, the Aegean, from his death. Theseus returns victorious to find himself king of Athens through his own father's grief-stricken suicide, a detail that gives the story of his triumph a somber, guilt-laden coda rather than a clean happy ending, and one later writers, including in the version preserved in Pseudo-Apollodorus's Bibliotheca, treat as part of the same continuous narrative rather than a separate misfortune.