Astyanax is the son of Herakles and Epilais who is one of the daughters of King Thespius. During his stay in the court of King Thespius, Herakles impregnated 49 out of the 50 of Thespius' daughters including Epilais who he lay with one night. The result of that particular encounter was Astyanax.
by Epilais he [Herakles] had Astyanax
- Apollod. Biblioteca 2.7.8