Assassin

Assassin (アサシン) is one of the seven normal Servant classes summoned for the Holy Grail War; Servants of this class are skilled at operating covertly, stealthy, and silently. This class is one of the weaker classes at fighting; as its members possess low attributes due to not having glorious legends as heroes; so it instead specializes in killing Masters rather than Servants. It is an oddity compared to the more formal knight classes who believe in fair duels and chivalry in combat; they will attack using the most unfair and favorable conditions; and with these conditions, it is possible for an Assassin to match and defeat other Servants. The Assassins' special class ability is ‘‘Presence Concealment”, which allows them to remain undetected when spying or preparing to ambush their targets.

Within the Fuyuki Holy Grail War system, it is said only the Hassan-i-Sabbah, summoned with the word "Assassin" as the catalyst, are applicable to be summoned due to them being the only true assassins to actually be recognized as Heroic Spirits. The Assassin in Fate/stay night, summoned by Caster, is an exception. He is an aberration in the system, as another Servant summoned him. While he has Presence Concealment, it is his own ability rather than a Class Skill, and he appears listed as "Assassin?" in the Fate/complete material III Servant section. Heracles, even within the Fuyuki system, has qualifications for all classes besides Caster, so it is unknown how he would fit into the role of Assassin. The rule originally remained in the Great Holy Grail War system, but Masters eventually managed to bypass it after the abilities of all ninteen Hassans became known. The Moon Cell Holy Grail War of Fate/Extra does not mention any such rule ever having been in place.