When Telegonus grew up, Circe sent him to find Odysseus, who by this time had finally returned to Ithaca from the Trojan War. On his arrival Telegonus began plundering the island, thinking it was Corcyra. Odysseus and his oldest son, Telemachus, defended their city and Telegonus accidentally killed his father with the spine of a stingray. He brought the body back to Aeaea and took Penelope, Odysseus' widow, and Telemachus, Odysseus' son, with him. Circe made them immortal and married Telemachus, while Telegonus made Penelope his wife. With Penelope, he was the father of Italus.
This is the story told in the Telegony, an early Greek epic which does not survive except in a summary, but which was attributed to Eugamon (or Eugammon) of Cyrene and written as a sequel to the Odyssey. Variants to the story are found in later poets: for example, in a tragedy by Sophocles, Odysseus Akanthoplex (which also does not survive), Odysseus finds out from an oracle that he is doomed to be killed by his son. He assumes that this means Telemachus, whom he promptly banishes to a nearby island. When Telegonus arrives on Ithaca, he approaches Odysseus' house, but the guards do not admit him to see his father; a commotion arises, and Odysseus, thinking it is Telemachus, rushes out and attacks. In the fighting he is killed by Telegonus.
In Italian and Roman mythology Telegonus became known as the founder of Tusculum, a city just to the south-east of Rome, and sometimes also as the founder of Praeneste, a city in the same region (modern Palestrina). Ancient Roman poets regularly used phrases such as "walls of Telegonus" or "Circaean walls" to refer to Tusculum.
See also The Odyssey.