Essence

Spirit Energy, alternatively translated as mana, is a kind of "life force" possessed by humans, elves, and other living things in the Monster Girl Encyclopedia world. It forms one half of a metaphysical yin-yang relationship with the opposite demonic energy of the monsters. Very similar to the Eastern concept of qi, spirit energy is lost by vigorous activity and magical spellcasting, while it is replenished by food, rest, and living in a healthy environment. Although monsters do not innately possess spirit energy, they all greatly hunger for it, and for some monsters it represents most of their diet. Since male semen contains highly concentrated spirit energy, this makes it the monsters' most delicious food.

General features
A mystical, metaphysical substance, spirit energy is generated by all living things except monsters and elementals. Small amounts are contained in organic foods like meat and plants, but it is intelligent life which produces it in the greatest abundance. Angels and elves both have more than humans, which provides them a limited degree of magical protection from monsterization. When spirit energy is expended, males replenish it naturally from within themselves. Females do the same but less quickly; they can also replenish their reserves by absorbing the the energy which is emitted passively by grass, plants, and other living things in the environment. While useful in human lands, this becomes a ruinous liability in demon realms, where absorbing demonic energy from the surroundings causes monsterization via spirit energy corruption.

Monsterization of women occurs when so much demonic energy is poured into the victim that her internal spirit energy production ability is extinguished, turning her into a creature of only demonic energy. This is impossible in men except for those of very particular psychologies; instead, for males demonic energy in their system spurs spirit energy production. Persistent exposure turns a man into an incubus, a creature of both demonic and spirit energy.

Although a few humans can command elemental magic, the vast majority of human mages use spirit energy magic. It is not clear whether magical skill is something that anyone can learn, or whether it's genetic; but by either nature or nurture, some people are better at it than others. Demonic magic looks very similar to spirit magic - to the extent that many people in the Monster Girl Encyclopedia world make no distinction - but the underlying energies are entirely different. Overuse of spirit magic - just like overuse of physical skills - causes exhaustion as spirit energy leaves the body; some monster girls, particularly akanames and beelzebubs, love licking mens' bodies precisely to get at the spirit energy contained in sweat. But all monsters prefer semen best of all, as the white stuff contains much more spirit energy than any other source.

When they have a husband, monster girls can become so enamoured with the taste of his spirit energy that they become addicted to it. Married monster girls also become extremely good at detecting their husband's spirit scent, and can essentially track him down wherever he is. For unattached monster girls who eat spirit energy, like succubi, a dietary supplement is brewed by golems wherein a limited amount of semen that the automata milk out of their victims can supply many monsters with nutritional, if foul-tasting, energy.