Image from Thomas.
Basic Description: Mohini is not a true avatar, rather a form that Vishnu assumes. She is called the enchantress because she is supernaturally beautiful and feminine. The desire to have her drives men mad. She is dangerous because it is so tempting to lust after her. For that reason Mohini is not worshipped, though respected, and no temples are named after her. There are very few visual representations of Mohini.
Mythology: The following are some of the central myths concerning Mohini:
Mohini saving Shiva: Shiva gave a demon the ability to turn anything he touched into ashes. However, the demon promptly exercised his new powers by chasing Shiva around the universe trying to touch him. No one could help Shiva because all were too afraid of Bhasmasura, this ash-demon. Mohini was the only exception. When Bhasmasura saw her, his desire for ashes turned into a desire for her. Mohini said that she would only marry someone who could match her in a dance contest. While both were very good dancers, she tricked him in the end. Mohini did a dance where she touched her head, and on imitating her Bhasmasura turned himself into a pile of ashes.
Shiva seduced: Shiva was curious about Mohini and asked Vishnu to show himself as such. Shiva though he could resist her charms. However, he was immediately enchanted. They joined hands to give birth to the god Ayyapan. Another account says that the child was Sastha.
Marrying Aravan: The Pandava and Kaurara clans had been fighting for days without any side making progress. The gods decided that the clan from whom a young soldier would sacrifice himself would win. Aravan the Pandava hero agreed under the condition that he would first be married. However, no father wanted to give his daughter in marriage if she were to be widowed so soon. Therefore, Vishnu decided to help. He camein the form of Mohini to marry Aravan and to allow the latter to fulfill his duty.
Clever Mohini: The gods and demons had been fighting, but decided to churn the ocean together and to retrieve the nectar of the bottom of the ocean. Many things came forth and were equally divided. However, when the nectar appeared the demons ran away with it without sharing. Soon they were arguing over it. Mohini came and offered to divide it up as long as the gods were included. The demons, overwhelmed by her beauty, agreed. Clever Mohini distributed the nectar to the gods first, with none left for the demons.
Moor, Edward. The Hindu Pantheon. Los Angeles: Philosophical research society, 1976.
Thomas, P. Epics, Myths and Legends of India. Bombay, India: D. B. Taraporevala Sons & Co. Private Ltd, 1961.