The picture is Caligula the Berserker from Fate Grand Order. he was a roman emperor that is one the bloodiest and evil like ruler Nero and Commodus. After reading the prophecy( in the dark Prophecy) and that the last emperor owns a fast white horse. I look up a fast white horse and roman emperor and got Incitatus( it means swift) who belongs to Caligula. He might not be the last leader of the Triumvirate Holding, but after looking he is the most likely. only after the the burning maze will i know if i was right. the title picture will probably not be how -if he is the last- he looks.
If you think it's a different one you may tell me.