Fate/Apocrypha

After the Greater Grail disappears from an earlier Holy Grail War, the Yggdmillennia clan reveals that it possesses the relic and declares independence from the Mage's Association. The response is a Great Holy Grail War: seven Servants of Black against seven Servants of Red, each backed by Masters with their own ambitions and legends. Jeanne d'Arc is summoned as Ruler to oversee the conflict, but the war quickly escapes the rules meant to contain it. Amid the magi and legendary heroes is Sieg, a homunculus created as nothing more than a source of magical energy who escapes in search of a life of his own. His small choices begin altering a war in which he was never meant to matter.