There was a king named Hieranyakshap who got blessed with the power of eternal life by a god named Lord Brahma
The king felt so powerful that he demanded that he should be the only person that should be worshiped and not any God including Lord Vishnu. Although his son Prahla kept worshiping Lord Vishnu.
He made Prahlad jump off a mountain
Trampled him with wild elephants
The King was so mad that he sentenced Prahlad to death, but to his surprised he survived all the murder attempts
He made Prahlad jump into a well
He even put him in a room with poisonous angry snakes
Finally the demon King made his sister Holika light herself on fire with Prahlad on her lap. She had a shawl for protection from the fire. But the wind flew the shawl to Prahlad and he was safe while Holika burned to death
The king was so angry that Prahlad had survived when suddenly Lord Vishnu appeared halve man halve lion and killed King Hieranyakshap atlast. That is the Story of Holi.