A powerful asura (demon)  once performed severe penance to propitiate Lord Shiva. Pleased with his devotion, Shiva appeared before him and asked him to make a wish.

"I wish that anyone upon whom I place my right hand is instantly reduced to ashes," the asura declared.

"So be it," said Shiva, unaware of the destruction this boon would soon bring.

The asura, now known as Bhasmasura (in Sanskrit, bhasma means "ashes"), began wreaking havoc. He reduced countless innocent people to ashes, and even attempted to use his boon against Shiva—the very Lord who had granted him the power. As a result, Shiva was also forced to flee from Bhasmasura.

Terrified, Indra and the other gods sought Lord Vishnu’s help. "O Lord," they pleaded, "Please save us from Bhasmasura."

Vishnu agreed to help. He transformed into a ravishing woman called Mohini (Sanskrit for "enchantress") and approached Bhasmasura. 

"Who are you, O beautiful one?" asked Bhasmasura.

Mohini smiled playfully. "I am Mohini. And who are you, O mighty one?"

"I am Bhasmasura, the most powerful being on Earth! I have been captivated by your beauty. Please marry me."

"I will marry you only on one condition: You must dance with me and copy every move I make."

Bhasmasura readily agreed. As Mohini danced gracefully, Bhasmasura mimicked her every step. Then, at the perfect moment, she placed her right hand on her head. Without thinking, Bhasmasura did the same.

And that was the end of Bhasmasura.

Sources

1. Om Namah Shivay (1997 Hindi TV series) by Dheeraj Kumar

2. Seethe (2010 Kannada TV series) by Krishan Sethi