As children, they fell asleep in the temple, and the sacred serpents licked their ears. From that moment, the twins were gifted with the power of prophecy. Cassandra was later cursed by Apollo so no one would ever believe her.
After Paris returned from his secret exile (his mother had dreamed of him as firebrand, and it had been foretold by prophets that he would be the ruin of Troy), Helenus and Cassandra went to him to dissuade him from his project of taking Helen of Sparta for his wife (as he had been promised by Aphrodite) warning him that he would bring destruction, but neither convinced him.
One night, Helenus went to a far temple of Apollo, but was intercepted by Odysseus who bad been informed by priest and seer Calchas that he would find him there. Threatening him, he obtained of him the keys to the fall of Troy: The Achaeans had to steal the Palladium (Athena's image) from the temple of Athena; the city would not fall until the Bow of Heracles loosed arrows against it (that bow was used by Philoctetes, a warrior left behind during the crossing of the Aegean Sea due to his wound); and that the war wouldn't end without the support of the son of Achilles, Neoptolemus.
Powers and Abilities
Precognition: Helenus possessed the power of prophecy.