Semele was a priestess of Zeus, and on one occasion was observed by Zeus as she slaughtered a bull at his altar and afterwards swam in the river Asopus to cleanse herself of the blood. Flying over the scene in the guise of an eagle, Zeus fell in love with Semele and repeatedly visited her secretly.Zeus' wife, Hera, a goddess jealous of usurpers, discovered his affair with Semele when she later became pregnant.