gacha

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 rule originally remained in the Great Holy Grail War system, but Masters eventually managed to bypass it after the abilities of all nineteen Hassans became known.