Some versions give a slightly more detailed version of the union of Shiva with Vishnu. One version tells that the asura Bhasmasura (Bhasma means 'Ashes') had so pleased Lord Shiva with his austerities that Shiva gave him a boon of anything he wished. So Bhasmasura asked for the ability to burn to ashes anything which he placed his hand over. No sooner had Shiva granted this, than Bhasmasura ran after the god, threatening to turn him to ashes.
A depiction of Harihara, (A fusion of Shiva and Vishnu), who is Ayyappa's father Shiva called to Lord Vishnu for help. He hide himself in a peepal tree as Bhasmasura ran here and there searching for the god. Vishnu became aware of the events, and decided that he would take the female form Mohini, "the Enchantress", and try to trump the asura's powers. When Bhasmasura saw Vishnu in this form, he was bewitched by her beauty. He earnestly tried to court her. So Vishnu instructed Bhasmasura to hold his hand over his head, and vow fidelity. With this act, Bhasmasura was reduced to ashes.
Vishnu found Shiva and explained the whole affair to him. Shiva asked if he too could see Vishnu in this female form. When Vishnu appeared thus, Shiva was overcome with passion, and united with her. The two gods thus became "Harihara Murthi", that is a composite form of Shiva and Vishnu as one god. From this union, Lord Sree Dharma Sastha was born.
http://femaleavatar.blogspot.com/search/label/Bhasmasur
