This emperor came to power after his predecessor, Carus, was killed by lightning in Persia, and he won the Battle of the Margus River to consolidate his power over rivals Carinus and Numerian.  
Contemporary sources about this man's reign include the text "Against the Nations" by the Christian writer Arnobius, and a failed rebellion against this man was led by Eugenius of Seleucia. Prior to coming to power, this man commanded forces in Moesia. This emperor's forces were responsible for an incident in Nicomedia called the "rivers of blood". This emperor made a text called the "Acts of Pilate" required reading in schools. This emperor came to power after his predecessor, Carus, was killed by lightning in Persia, and he won the Battle of the Margus River to consolidate his power over rivals Carinus and Numerian.  This emperor made Galerius his caesar and gave the title of co-emperor to Maximianus. After building a palace at Spalato, now known as Split, this emperor who ordered the largest anti-Christian persecution famously grew lettuce. For 10 points, name this Roman emperor who established the Tetrarchy and peacefully retired. 
Contemporary sources about this man's reign include the text "Against the Nations" by the Christian writer Arnobius, and a failed rebellion against this man was led by Eugenius of Seleucia.
Diocletian [or Diocles] 