Assassin (アサシン, Asashin?) is the Assassin-class Servant of Caster in the Fifth Holy Grail War of Fate/stay night. He is an irregular Servant that shouldn't exist according to the rules of the Holy Grail War, but Caster's unique nature as a Servant summoning a Servant twists the process. The true Servant, one of the Hassan-i-Sabbah, is summoned as True Assassin in the Heaven's Feel route. He is one of the Servants of the Protagonist of the Grand Order conflicts of Fate/Grand Order.