During the Trojan War, the Greeks captured Helenus, a prophet, and tortured him until he told them under what circumstances they could take Troy. Helenus said they would win if they retrieved Heracles' arrows (which were in Philoctetes's possession); steal the Trojan Palladium (they accomplished this with the Trojan Horse) and persuade Achilles' son Neoptolemus to join the war. Neoptolemus was hiding from the war at Scyros but the Greeks retrieved him. Alternatively, he told them that they could win if Troilius, Helenus' half-brother, son of Apollo and Hecuba, was killed before he turned twenty. Achilles ambushed Troilius and his sister, Polyxena.
After his parents were killed by the Greeks in Trojan War, Helenus was enslaved by Neoptolemus and taken to Epirus. After Neoptolemus was killed by Orestes, Helenus inherited part of his kingdom and married Andromache, another slave.
Virgil III, 295, 334, 374.