 Some of the ancient sources detail plots that Galerius made on Constantine 's life in the months following Diocletian 's abdication . They assert that Galerius assigned Constantine to lead an advance unit in a cavalry charge through a swamp on the middle Danube , made him enter into single combat with a lion , and attempted to kill him in hunts and wars . Constantine always emerged victorious : the lion emerged from the contest in a poorer condition than Constantine ; Constantine returned to Nicomedia from the Danube with a Sarmatian captive to drop at Galerius ' feet . It is uncertain how much these tales can be trusted . 
