Bhasmasura was a very powerful asura (demon). He was very big and strong but at the same time, he was also very foolish.

Bhasmasura prayed to Shiva. Pleased with his devotion, Shiva decided to grant him a boon. When Shiva asked, Bhasmasura said,  "Lord, please grant me the wish that whichever person, place or thing I place my right hand on, is reduced to ashes." Lord Shiva granted him the boon and Bhasmasura was very happy.

Having got the boon, the wicked Bhasmsura tried the boon on Shiva himself. As a result, Shiva started running from Bhasmasura to save himself. Seeing Shiva's plight, Lord Vishnu decided to help him. He changed himself into a beautiful young maiden, called Mohini, instead. He was attacked by Mohini's beauty and asked her to marry him. Mohini agreed to marry him, but she had a condition. She asked Bhasmasura to touch his head and promise her that he would not marry again if she became his wife. Bhasmasura agreed. As soon as the foolish demon touched his head, he was reduced to ashes. That is how Vishnu succeeded in saving Shiva from Bhasmasura.