Lord Vishnu gave demon king Hirianyakashyap powers that he would not be killed by man or animal, at day or night, inside or outside, above or on the ground.
Hirianyakshyap said that he was the only one who would be worshipped, not god. His son Prahlad continued to worship Lord Vishnu. This made his father very angry.
Hiranyakshyap ordered elephants to trample his son but the elephants didn't trample Prahlad.
Holika is Hiranyakshyap's sister. Holika had a robe that made it so she didn't burn in fire. So she made Prahlad sit on her lap in the fire. When the fire started Prahlad magically took the claok of of Holika and put it on himself. Holika died in the fire.
Lord Vishnu came back as a half man and half lion to kill Hairanyakshyap.