S1E5: Stars' Banquet

Shortly before the battle in the canyon, in a mansion situated in a swamp, a lone mercenary attempted to summon a Servant. His codename is Sigma. He holds no desires for this world, nor any intention of leaving anything behind. Chosen by the fake Holy Grail for this empty shell of a man was a Servant so singularly unique that it could never be summoned in a normal Holy Grail War...