The Hariharaputra-birth narrative (Puranic tradition)
During the deva-asura churning of the ocean, Vishnu assumed the female-form Mohini-avatar; Shiva consorted with Mohini — and from this unique union was born Hariharaputra Ayyappa. The demoness Mahishi's boon was that only a son of Vishnu + Shiva could slay her — Hariharaputra was born expressly to fulfil that condition.
