Star of Calvaria

The ring of an angel deployed in the stratosphere, which blazes by converging the light of the Sun. A reflector made by magecraft. The blade of light reaching fifty kilometers in length and ten kilometers in width that is beamed towards the earth can be compared to an aurora with mass, as well as to a guillotine of light.
It’s a High-Thaumaturgy devised by Ciel to defeat the Ancestor who was her instructor of the sword, and their Idea Blood is apparently used for its activation.
After it was beamed at full power against Arcueid, another shot using [Ciel’s] remaining mana was performed, vitally turning the tables at the very end of the fight.

Tsukihime Material I: Tsukihime Work Glossary