Heaven's Fall

The Reverse Moon (天の逆月). Not the Grail of Heaven (天の杯), but Angra Mainyu, the core of the Fuyuki Holy Grail.

While it is weak compared to the maelstrom of prana that is the Einzbern Grail, what it can do is grant wishes. In a way, it is a small but "true" Holy Grail.

The foundation of the looping world in hollow ataraxia, where it appears as a black moon, hence the name.