In the olden days, giant demons were called daityas.
Hiranyakashyap was the king of the daityas and wanted to conquer the world
and be worshipped by everyone.
Hiranyakashyap's son, Prahlad, was a devotee of Lord Vishnu.
Hiranyakashyap did not like this but Prahlad was unmoved in his devotion.
So, Hirnyakashyap decided to kill Prahlad. He tried to get snakes to
crush Prahlad, but Prahlad prayed to Vishnu and was saved. Then
Hiranyakashyap tried to throw Prahlad from a cliff, but Prahlad escaped.
Finally, Hiranyakashyap called his sister Holika, a she-demon, to kill
Prahlad. Holika had been given a boon that fire could not destroy her. She
decided to take Prahlad in her lap and sit on a burning pyre.
As the flames engulfed them Prahlad called aloud for Lord Vishnu's
help. To everybody's surprise, the fire slowly consumed Holika and she
burned to death. Prahlad emerged safe and sound.