In Greek mythology, Helenus was a Trojan prince and seer, son of Priam and Hecuba and twin brother of Cassandra. Homer calls him far the best of augurs among the Trojans. Captured by Odysseus after losing Helen to Deiphobus, he revealed the oracles by which Troy could be taken; he survived the war, settled in Epirus with Neoptolemus's household, married Andromache in many accounts, and prophesied Aeneas's Italian destiny at Buthrotum.
Adapted from Wikipedia, "Helenus of Troy" · CC BY-SA 4.0
I.Prophet and warrior
Early sources say snakes licked the ears of Helenus and Cassandra, granting them prophecy; others say Cassandra taught him, but his predictions — unlike hers — were believed. He foresaw that if Paris brought a Greek wife home, the Achaeans would destroy Troy. In the Iliad he advises Hector to challenge the best of the Achaeans, leads a battalion with Deiphobus, and is wounded through the bow-hand by Menelaus.
Hyginus still lists him among famous augurs beside Cassandra. He must be told apart from Helenus of Ithaca, a minor figure in Homeric catalogues, and from other rare namesakes.
II.Capture and the fall of Troy
After Paris's death Helenus and Deiphobus vie for Helen; Deiphobus wins and Helenus withdraws to Mount Ida. Calchas tells the Greeks that Helenus knows the city's protecting oracles; Odysseus waylays him. Under coercion Helenus discloses the conditions of capture: fetch the bones of Pelops, bring Neoptolemus (and in fuller cycle accounts Philoctetes with Heracles' bow) into the war, and steal the Palladium from within Troy.
Those revelations clear the path for the horse stratagem and the sack. Some late chronicles instead honor him as a peace-party Trojan freed by Agamemnon with Cassandra.
III.Epirus and Aeneas
Apollodorus sends Helenus overland with Neoptolemus to Molossia, where he founds a city and receives Deidamia as wife. Pausanias has him marry Andromache after Neoptolemus's death at Delphi, guard the dead king's children, and father Cestrinus. Virgil's Aeneid places him and Andromache at Buthrotum, a little Troy, where he prophesies Aeneas's founding destiny before the Trojans sail on.
Other strands give him charge of Hecuba's burial at Kynosema, or — in Geoffrey of Monmouth — a chained captivity in Greece that later feeds the Brutus legend. He remains the type of the Trojan seer who outlives the city he helped bring down.