FSN Magecraft Notes

Blood Fort: A type of bounded field known also as a "soul eater". It melts the bodies of any non-mage human within it and forcibly collects the exposed souls.

Second Factor: Soul. Only one mage has ever proven their existence, so most mages consider them theoretical. Servants can eat these (and also the mind) to restore power if their mages aren't strong enough to sustain them properly or they just want a major durability boost. It just restores their mana.
Third Factor: Mind.
Fourth Factor: Not yet mentioned.
Fifth Factor: ??? Nobody can look at it or know what it is. Rin's vision even shuts off when she summons it.