Assassin is summoned and refuses Caster's request to guard Fuyuki Temple. She uses a command spell and Assasin must obey. He faces Saber as a mid-boss, by order from Caster's 2nd command spell. In his final battle, Berserker appears and knocks him down. Caster uses her 3rd command spell to order him to stand up and fight. Now that he regained his free will, he kills Caster and faces Berserker.
Now without Mana supply, he thinks about the Grail War before he disappears.