In the Holy Grail War, a Heroic Spirit's magical energy is typically provided by their Master, but there are some who satisfy themselves by consuming human souls.
『Blood Fort Andromeda: Other Seal, Blood Temple』
This bounded field created by a magical circle is a type of absorption curse that forcefully withdraws magical energy of humans who are inside the circle. If the circle is activated to its full power, it has the power to even dissolve human flesh.
In a Holy Grail War, it is a common knowledge that a Master is supposed to supply mana for the Heroic Spirit, but there exists a Heroic Spirit that consumes a human's soul to negate such obligations.
『Outer-Seal, Blood Temple
Blood Fort Andromeda』
A Bounded Field that can only be created by setting up multiple Magic Circles at various points, normal humans inside the barrier are weakened and their Magical Energy were sucked out from them,
Once activated, it will dissolve all victims inside the barrier, continuously.