Bloodfort Andromeda

Outer Seal: Blood Temple.
A boundary field of blood. Any humans captured within it will be dissolved.
The dissolved humans are reduced to red blood, which is consumed by the boundary field's user. Its primary purpose is the absorption of magical energy.
It was used once in every route, and by the end of the game was like an old familiar friend.
Since the boundary field is in the shape of a dome, the school building looked as if it was captured within an enormous eyeball.
Naturally, from outside the boundary field the red sky and dome cannot be seen at all. It's just like Rin said: any boundary field that reveals its strangeness to the outside world is third-rate at best.
Also, because the Bloodfort seriously damages the leylines where it is used, it cannot be used on the same land twice in succession.

Fate/side material: Fate Encyclopedia