Aethra in Wikipedia

In Greek mythology, Aethra or Aithra (Ancient Greek: Αἴθρα, the "bright sky"[1]) was a name applied to three individuals: Æthra was a daughter of King Pittheus of Troezen and, with the king Aegeus of Athens - or in some versions, Poseidon -father of Theseus. Ægeus went to Troezen, a city southwest of Athens that had as its patrons Athena and Poseidon, where Pittheus got Ægeus drunk on unmixed wine and put him to bed with his daughter. Following the instructions of Athena in a dream, she left the sleeping Ægeus and waded across to the island of Sphairia that lay close to Troezen's shore. There she poured a libation to Sphairos, Pelops' charioteer, and was possessed by Poseidon in the night. When she was thus doubly pregnant, Ægeus decided to go back to Athens. Before leaving, he covered his sandals, shield and sword under a huge rock, that served as a primitive altar to Strong Zeus, and told her that when their son would grow up, he should move the rock and bring his weapons back. Aethra did as she was told, and Theseus, recovering the weapons that were his birthright, grew to be a great hero, killing the Minotaur, among other adventures...

Link: https://en.wikipedia.org/wiki/Aethra_(Greek_mythology)