Of all the avataras Mohini is the only female avatar. But the most beguiling of them all. She is portrayed as an enchantress, who maddens lovers, sometimes leading them to their doom. Unlike the Dasavataras, which appear on Earth during a certain period, Vishnu takes up Mohini avatara during many time periods. In the original text, Mohini is referred to as simply an enchanting, female form of Vishnu. In later versions, Mohini is described as the maya(illusion) of Vishnu (mayam ashito mohinim).
Almost all her tales has that element of slyness. Most of which were the ones leading the asuras (the bad guys) to doom. Bhasmasur was one such asura. Bhasmasur was a devotee of Lord Shiva (Well, Lord Shiva had no restriction upon who could worship him. He was known as Bholenath – easily pleased). He used to perform long penances in order to please Shiva. Shiva being pleased with his austerities, granted him one wish. Bhasmasur, asked him for the one obvious wish – immortality. However, this was out of Shiva’s ‘pay-grade’. So, he asked for the next coolest wish – the license to kill. Bhasmasur asked that he be granted the power that anyone whose head he touched with his hand should burn up and immediately turn into ashes (bhasma).
Well, so far things were going fine for Shiva. Bhasmasur, now sees the beautiful consort of Shiva – Parvati. A pervert and wicked asura as he was, wanted to possess her and marry her. He, thenceforth tries using his newly granted boon on Shiva himself (one piece of rotten asura he is). Shiva, being bound by the ‘contract’ had no power to take back his grant. He fled, and was chased by Bhasmasur. Wherever Shiva went, Bhasmasur chased him down. Somehow, Shiva managed to reach Vishnu to seek a solution to this predicament. Vishnu on hearing Shiva’s problem, agreed to help him out.
Vishnu took up the form of Mohini and appeared in front of Bhasmasur. Mohini was so exceedingly beautiful that Bhasmasur immediately fell in love with Mohini (This is what years of austerity does to you). Bhasmasur asked her (Mohini) to marry him. On a side note, the asuras of the vedic times were real gentlemen. The only way to be with a woman was to marry them. Anyways, Mohini asked her out on a dance, and would marry him only if he could match her moves identically. Bhasmasur agreed to the match and hence they started dancing. The feat went for days at an end. As Bhasmasur matched the disguised Vishnu’s move for move, he began to let his guard down. While still dancing, Mohini, struck a pose where her hand was placed on top of her own head. And Bhasmasura, whose eyes were constantly fixed upon Mohini’s beautiful face, completely forgot about Lord Shiva’s boon, and put his hand on his head too and turned into ashes.