Theodicy (Fate/Zero, SI!Caster)

Summoned into the body of the infamous murderer, Gilles de Rais, Caster of the Fourth Holy Grail War, destined to bring devastation and suffering to the city of Fuyuki, our protagonist attempts to chart a way back home, away from the dangerous and unpredictable world of Fate. But each Servant must have a Master, and so, our unfortunate SI must ally with an ambitious young mage, whose goals only apparently align with his own.