There was once a proud and wicked King called King Hiranyakashipu. He considered himself the supreme God and commanded his subjects to obey him and only him.
However, he had a son called Prahlad who was a devotee of Lord Vishnu. One day, the King called him son and asked:
"Now, my son, you are very clever. But what is the most important thing you have learned at school?"
To this, Prahlad replied:
" That you are the King. But Lord Vishnu is God, and he is greater than everyone"
This angered King Hiranyakashipu. Despite commanding him to worship him only, Prahlad bluntly refused. Thus, the King decided to punish him.
He tried a lot of things- throwing him off the cliff, throwing him in front of a giant elephant, and a pit filled with poisonous snakes. However, Prahlad couldn't be killed as Lord Vishnu protected him every time. The soldiers even tried to kill him with their swords but of no use.
Left with no option, King Hiranyakashipu asked his witch sister Holika to do something. Holika hatched an evil plan in order to kill Prahlad.
She lit a huge bonfire in front of the palace and asked Prahlad to walk with her in the fire. Holika was blessed with immunity to fire, never to be consumed by its flames. She decided to leave Prahlad in the fire. However, this backfired as Lord Vishnu protected him while Holika burnt.
After that, Lord Vishnu took the form of half a lion and half a human called Narasimha to kill the evil King Hiranyakashipu. Thus, Holi symbolises the victory of good over evil.