Prefect Simon Constantin. (Picture is fanart.)
Prefect Simon Constantin was the son of Grand General Douglas Constantin, who would later defect from the newly created Terran Overlord Government with the Renegade Legions. At the time of the proclamation of TOG (6680), Simon was Prefect of the 182nd Pretorian Heavy Infantry Legion, stationed on Terra.
In the chaos after the bombing of the Senate and the sudden end of the Republic, the loyalties of the remaining Legions on Terra were divided as the civilian populace thronged the streets protesting the proclamation. Refusing the Caesar´s orders to fire on the unarmed protestors, Constantin´s Legion was attacked by Loyalist forces and forced to defend itself. Outnumbered and outgunned, they nevertheless fought their way through the enemy lines up the New Roman Capitol and came within an inch of capturing Caesar Ivanolo Buntari. The usurper was saved in the nick of time by a Loyalist Defensor fighter squadron.
Soon after, the remnants of Constantin´s Legion were defeated. Every surviving man and officer was beheaded, and Simon Constantin tried for treason and publicly executed. It has been widely speculated that his death and show trial were among the factors that precipitated the defection of Grand General Douglas Constantin and the Renegades.