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Penthilus (//; Ancient Greek: Πένθιλος) is the illegitimate or legitimate son of half-siblings Orestes and Erigone in Greek mythology. Penthilus' grandmother was Clytemnestra. His maternal and paternal grandfathers were Aigisthos and Agamemnon respectively. Orestes killed both Clytemnestra, who was his own mother and Aigisthos. Erigone is said to have hanged herself or married Orestes after Orestes' first wife, Hermione died. Orestes was ruler over much of the Peloponnese and died of a snakebite at age 70. One story says that as a child, Penthilus was torn apart and devoured by wolves in the Taygetus mountains, near Sparta. His father established a festival of mourning, the so-called Penthilia in his honour.
According to Pausanias, Penthilus grew up and founded a city either on Lesbos or in Thrace. He had a son named Damasias who in turn fathered Gras, the founder of the city Aeolis, between Ionia and Mysia.