[Kristen]

I have a theory. This is the same world as Fate/Stay night. I don't think anything is particularly spoilerish below, but read at your own risk.

Spoiler for FSN:

Denizens are simply heroic spirits summoned to this world. So while Kings of Guze are provided power of existence via the Flame Haze, the Guze no Tomogara need to replenish their power of existence/mana. So they either need to sleep to replenish it slowly, eat humans to gain it, or get it through the semen of a magus.

Makes me wonder if Magery Dow really was Keisaku's first. >.>