Half-divine son of Poseidon by a mortal mother (Calyce, Scamandrodice, Harpale, or Phylonome across competing variants); king of Colonae in the Troad; first significant Trojan-side casualty in the Greek landing before Troy. The divine endowment of invulnerability to ordinary weapons constrains the killing-method to non-piercing improvisation — Achilles strangles him with his helmet-strap in the dominant Ovidian tradition. Transformed into a swan at the moment of death per Ovid, the eponymous-bird metamorphosis honoring the swan-name. Distinct from the other Cycnus figures in Greek myth: Cycnus son of Ares (killed by Heracles, treated in Hesiod's Shield), Cycnus son of Apollo and Thyrie (the swan-eponym narrative in Antoninus Liberalis 12), and the Ligurian Cycnus mourning-friend of Phaethon — all separate figures sharing only the bird-name.