True Assassin is an exceptionally dangerous Servant who only makes an appearance in Heaven's Feel. The designation of "True" is meant to distinguish this Assassin from another Servant that has usurped his place in the Fifth Grail War. All True Assassins share a collective identity and have appeared in every Holy Grail War throughout history. They are infamous for an ebony and tattered costume, as well as for concealing their identity behind an ashen skeletal mask. [Edited from Wikipedia]