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From Old Greek Stories by James Baldwin:
Soon after this, Apollo took the little Asclepius in his arms and carried him to a wise old schoolmaster named Cheiron [the centaur], who lived in a cave under the gray cliffs of a mountain close by the sea.
And Asclepius proved to be a wise child, gentle and sweet and teachable, and among all the pupils of Cheiron he was the best loved. He learned the lore of the mountains, the woods, and the fields. He found out what virtue there is in herbs and flowers and senseless stones, and he studied the habits of birds and beasts and men. But above all he became skillful in dressing wounds and healing diseases, and to this day physicians remember and honor him as the first and greatest of their craft. When he grew up to manhood his name was heard in every land, and people blessed him because he was the friend of life and the foe of death.
As time went by, Asclepius cured so many people and saved so many lives that Pluto, the pale-faced king of the Lower World, became alarmed. "I shall soon have nothing to do," he said, "if this physician does not stop keeping people away from my kingdom."
And he sent word to his brother Jupiter, and complained that Asclepius was cheating him out of what was his due. Great Jupiter listened to his complaint, and stood up among the storm clouds, and hurled his thunderbolts at Asclepius until the great physician was cruelly slain. Then all the world was filled with grief, and even the beasts and the trees and the stones wept because the friend of life was no more.
When Apollo heard of the death of his son, his grief and wrath were terrible. He could not do anything against Jupiter and Pluto, for they were stronger than he, but he went down into the smithy of Vulcan, underneath the smoking mountains, and slew the giant smiths who had made the deadly thunderbolts.
Then Jupiter, in his turn, was angry, and ordered Apollo to come before him and be punished for what he had done. He took away his bow and arrows and his wonderful lyre and all his beauty of form and feature, and after that Jupiter clothed him in the rags of a beggar and drove him down from the mountain, and told him that he should never come back nor be himself again until he had served some man a whole year as a slave.
And so Apollo went out, alone and friendless, into the world, and no one who saw him would have dreamed that he was once the sun-bright Lord of the Silver Bow.
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... Apollo spent his year of slavery in service to Admetus; you can read more about that at Wikipedia: Admetus.