When Zeus struck down Apollo's son Asclepius with a lightning bolt for resurrecting Hippolytus from the dead (transgressing Themis by stealing Hades's subjects), Apollo in revenge killed the Cyclopes, who had fashioned the bolt for Zeus. Apollo would have been banished to Tartarus forever for this, but was instead sentenced to one year of hard labor, due to the intercession of his mother, Leto. During this time he served as shepherd for King Admetus of Pherae in Thessaly. Admetus treated Apollo well, and, in return, the god conferred great benefits on Admetus.
Who is Apollo's son?
Asclepius