Helenus was the son of King Priam and Queen Hecuba of Troy, and the twin brother of the prophetess Kassandra. According to legend, Kassandra, having been given the power of prophecy by Apollo, taught it to her brother. Like Kassandra, he was always right, but unlike her, others believed him.
Helenus was part of the Trojan forces led by his brother Hektor that beat the Greeks back from the plains west of Troy, and attacked their camp in the Iliad. When the Myrmidons led by Achilles turn the tide of battle and Hektoris killed, foreshadowing Troy's imminent fall, Helenus - like most of the greatest heroes - survived the poem.
In the final year of the Trojan War, Helenus vied against his brother Deiphobus for the hand of Helen of Troy after the death of their brother Paris, but Helen was awarded to Deiphobus. Disgruntled over his loss, Helenus retreated to Mount Ida, where Odysseus later captured him. He told the Greek forces—probably out of his disgruntlement—under what circumstances they could take Troy. He said that they would win if they stole the Trojan Palladium, brought the bones of Pelops to Troy, and persuaded Neoptolemus (Achilles' son by the Scyrian princess Deidamia) and Philoctetes (who possessed Heracles' bow and arrows) to join the Greeks in the war. Neoptolemus was hiding from the war at Scyrus, but the Greeks retrieved him.