The story of the Mohini Avatar of Vishnu is found in the Srimad Bhagavad Purana and numerous other Hindu scriptures. At the end of Samudra Manthan or Churning of Ocean, Dhanvantri, god of medicine, appeared with a pitcher full of nectar of immortality or Amrita. The pot of Amrita became a cause of fierce fight between the Devas (celestial beings) and Asuras (demons).
Jayant, the son of Indra, the king of celestial beings, grabbed the pot of Amrita and ran away. The demons chased him and the fierce fight continued.
The demons finally defeated the celestial beings and got hold of the nectar and started celebrating.
The celestial beings then appealed to Bhagavan Vishnu to help them.
Vishnu then took the form of Mohini, a celestial damsel. There was no other woman who was so beautiful and enchanting in the entire universe. She distracted the demons and took the Amrita and promised that she will distribute it equally among Asuras and Devas. But she distributed it only amongst the celestial beings.
Rahu, one of the demons, disguised himself as a celestial being and drank some of the Amrita. Sun God and Moon God soon realized that the one sitting between them was a demon. They informed it to Mohini. Before the nectar could pass his throat, Mohini cut off his head with the divine discus, the Sudarshana Chakra.
Numerous other minor stories are found in various other Puranas and in folklore.