Why did Hercules obey Eurystheus?

The evil goddess Hera took a dislike to Hercules and sent him an attack of madness. In this attack, he killed all his children and his brother’s children. Having come to his senses, the hero went to Apollo to ask what to do. Apollo ordered Hercules to return to his homeland at 12 years old and serve the cowardly king Eurystheus and complete 10 of his tasks. Only in this way can a demigod obtain immortality.

Hercules had to complete not ten, but twelve tasks, because the cowardly king did not count two exploits.