The enchantress Mohini was a form that VISHNU took during the churning of the MILK OCEAN. The demons and gods had cooperated in churning the ocean to produce the nectar of immortality (AMRITA). Knowing that the demons would want to seize the nectar for themselves, Vishnu assumed the form of MOHINI, mesmerizing the demons with her beauty while she served the nectar to the gods alone. As only the gods were now immortal, when the demons attacked they were routed, and the world was once again in the hands of the gods.
Further reading: Guruseva Dasi, Churning the Milk Ocean: A Young Reader’s Edition of the Classic Story from the Puranas of Ancient India (LaCrosse, Fla: Bhavani Books, 2002); Cornelia Dimitt, and J. A. van Buitenen, eds. and trans., Classical Hindu Mythology A Reader in the Sanskrit Puranas (Philadelphia: Temple University Press, 1978).
Encyclopedia of Hinduism. A. Jones and James D. Ryan. 2007.