In April 175, Avidius Cassius, Governor of Syria, declared himself Emperor following rumours that Marcus Aurelius had died. Having been accepted as Emperor by Syria, Palestine and Egypt, Cassius carried on his rebellion even after it had become obvious that Marcus was still alive. During the preparations for the campaign against Cassius, the Prince assumed his toga virilis on the Danubian front on 7 July 175, thus formally entering adulthood. Cassius, however, was killed by one of his centurions before the campaign against him could begin.
Commodus subsequently accompanied his father on a lengthy trip to the Eastern provinces, during which he visited Antioch. The Emperor and his son then traveled to Athens, where they were initiated into the Eleusinian mysteries. They then returned to Rome in the Autumn of 176.