Fate/EXTELLA: The Umbral Star
Now, the Servant Nero Claudius and her Master have won the Holy Grail War and have obtained the universal wishing mechanism, the Holy Grail.
They also obtained the Regalia, a ring that proves their kingship. With it, they rule over their once-enemy Servants and start a new beginning on the moon's new world.
But a new enemy has appeared before them. And this enemy has something that shouldn't exist: another Regalia.