When Hercules entered the temple at Delphi he was very dirty.  Pythia refused to give an oracle to Hercules as a result of his dirtiness.  Hercules became very angry at the priestess, and he snatched the holy tripod of the priestess and took it away from the temple.  Hercules planned to built another temple using the holy tripod of the priestess.

Pythia called upon Apollo and told him of what had happened, and of Hercules plans to build another oracle.  Apollo became enraged and hunted down Hercules finding him in the Peloponnese.  Apollo attacked Hercules trying to kill him but he could not, nor could Hercules kill the god Apollo.  For three whole days the two combatants fought each other and it is said that no one would have won the battle, and the battle would have lasted for all time.  

The god Zeus appeared at the end of the third day and he brought conciliation between the two combatants.  Zeus told them both that the solution was to allow Hercules to hear from the Oracle, and in turn Hercules would return the holy tripod and not build a new temple.  Both Hercules and the god Apollo agreed to the decision that Zeus had made.

At the oracle Pythia told Hercules that there was only one way that his sin could be forgiven she said, "Go to your uncle Euristhea King of the Mycenaeans and perform twelve tasks that he asks of you, then your sin will be forgiven."

Hercules uncle became frightened when he saw Hercules arrive at his palace, he thought that Hercules had come to steel his kingdom from him.  Euristhea gave Hercules twelve tasks that he thought were both insurmountable and dangerous, his uncle felt that surely Hercules would die in these tasks and his kingdom would then be safe.

Within twelve years Hercules completed these twelve feats, and the sin that plagued him for many years was finally forgiven.