Penthilus was the illegitimate or legitimate son of half-siblings Orestes and Erigone. Penthilus' grandmother was Clytemnestra. His maternal and paternal grandfathers were Aigisthos and Agamemnon respectively. Orestes killed both Clytamnestra, who was his own mother and Aigisthos. Penthilus' mother is said to have hanged herself or married Penthilus' father after Penthilus' father's wife, Hermione died. His father 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.