The Inca people believed in a great many gods and goddesses. The Inca believed that these gods would protect them. But the Inca had been losing battles in war. The had lost a great many warriors. The Sapa Inca believed that the gods were angry with the Inca. The Sapa Inca decided to sacrifice a great many people so that the gods would be happy again.
Pachacuti could not believe his father was considering sacrificing his own people. Pachacuti was a good son. He had always done what his father had told him to do. But this was wrong. He could not allow this to happen. Pachacuti gathered the warriors who were left, and led them into battle. He convinced the warriors that the gods had told him that this time they would be victorious. And they were.
After the battle was over, Pachacuti returned to the capital city. By the time he returned, the gossips had been busy telling everyone that his father had planned to sacrifice a great many people, mostly nobles, including children. The sacrifice had to be a good one, otherwise what good would it do? Pachacuti told his father that it was time his father retired. Pachacuti crowned himself the new Sapa Inca. All the nobles and all the people were glad. They called him the Hero Pachacuti.
That was the beginning of the Inca Empire.