This is a completely new scenario for Rome II, set in 311 A.D. Also available on Steam Workshop. The Tetrarchy of Diocletian Augustus divided the Empire in 293 A.D. Not long after, tensions arose between the various Emperors, all vying to become sole sovereign of the Roman world.
This is the main mod pack. Put it in your Rome 2/data directory and activate it in the Rome 2 Mod Manager (from the Rome 2 launcher).