One of the main reasons for the incarnation of Hindu God Ayyappa was to annihilate the demon Mahishi. The popular belief is that the union of Hindu God Shiva and Vishnu in his Mohini incarnation gave birth to Ayyappa. The reason for this unique union was a boon acquired by Mahishi.
Mahishi was the sister of Demon Mahishasura who was killed by Goddess Durga, who came up on earth with the energy of the Trimurtis and other Devas. Mahishasura had received the boon that no man could kill him but he was eventually killed by Goddess Shakti in her Durga form.
Demon Mahishi went a step ahead and performed intense tapas and austerities to please Brahma. Finally, Brahma relented and as a boon Mahishi asked that a son born to Shiva and Vishnu should only kill her.
She asked this boon as Vishnu and Shiva had got better of all the boons that the demons had received earlier. Now it was impossible for a son to be born to Shiva and Vishnu as they were of same gender.
Vishnu had taken the form of Mohini to rescue Amrut got during the Samudra Manthan. Shiva once expressed a desire to see the Mohini form which had enamored both the Devas and Asuras. Vishnu again appeared in the form of Mohini. Thus Shiva and Mohini united to give birth to Ayyappa.
The king of Pandalam in Kerala found Ayyappa in the forest and he was brought up as a prince and He later annihilated Mahishi.