The accounts of the end of Theseus are many and inconsistent. They say he was kept a prisoner until Heracles restored him to the light of day, but the most plausible account I have heard is this. Theseus invaded Thesprotia
to carry off the wife of the Thesprotian king, and in this way lost the greater part of his army, and both he and Peirithous （he too was taking part in the expedition, being eager for the marriage） were taken captive. The Thesprotian king kept them prisoners at Cichyrus.