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.
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.
Spirit is the embodiment of life energy innately possessed by members of all races barring Monsters, and is possessed in large amounts by human males. Spirit is similar to what is commonly known as Demonic Energy and as such shares a plus/minus relationship with it. If there is more Spirit, there would be less Demonic Energy. One might think of it as a Yin and Yang relationship - if one exists more than the other the other becomes lessened as a result, ie Inversely proportionate.
Human mages' magic and elven magic utilize this Spirit themselves. However, it is often called "Magic" by humans and elves, and it is often confused with Demonic Energy.
Some monsters use this Spirit Energy as food, and obtain it through intercourse. By absorbing the spirit obtained during intercourse into their bodies, it is then converted into "Demonic Energy". One might view Demonic Energy as their life-force.
Human males possess the ability to produce this spiritual energy, and as such are not harmed when monsters take this from them, as it can be regained through food or rest.
Hence, the ability to produce spiritual energy and store it inside the body would classify the incubi as human instead of monster.
A vital energy possessed by humans, and various other races besides monsters. Men especially have a lot of it. When human wizards or elves use magic, the energy used is the same thing as spirit energy, but the monsters are the only ones who actually refer to it as such. It is generally called magical energy. It is extremely similar to the demonic energy of monsters . If spirit energy is the plus, then the demonic energy of monsters is the minus. They are related like two sides of the same coin.
A Feast for Monsters
The monsters feed on spirit energy which they get from men through sex. Much spirit energy is included in the saliva and semen of men. Besides swallowing it down and absorbing it along with everything else using their mouth, they most of all enjoy the method of using their vagina to squeeze out semen. As far as the monsters are concerned, spirit energy is a treat far better than any other food. When they gain spirit energy, their thoughts melt away as they experience a powerful rush of joy and ecstasy. Spirit energy that they've absorbed in this way is then converted to demonic energy, which will be discussed later, inside their body and stored.
The taste of the spirit energy is heavily influenced by the state of the monster when she takes it in. The stronger her desire is for the man who will become her prey, the more exquisite the taste of the spirit energy becomes, especially the taste of the spirit energy of a man that instinct has recognized as her husband. It tastes so much sweeter to her than anything else that she wouldn't even bother feeding on another man's spirit energy again. Most of the monsters become a prisoner of their husband's spirit energy. It is said that many of them even fall into a state that could be called “semen addiction.” Also, there are some medicines made from spirit energy for nutritional supplement. But for the reason described above, it seems that it's not something that tastes very good to them at all. Furthermore, human men are always giving off minuscule quantities of spirit energy. By sensing it, the monsters can infer the presence of a human man. They are especially sensitive to the spirit energy of their own husband. They are always able to sense his location and track him down. It is to such a degree that no matter where he tries to hide, she can always find him no matter what.
Recovery of Spirit Energy
The spirit energy that dwells within the body of a human, besides being stolen away by monsters, can also be spent through casting spells. Even if spirit energy is lost for one of these reasons, it never results in death, though it may cause the person to feel fatigue, or pass out. In the case of human men, they have the power to create spirit energy inside their body. It is possible to recover their spirit energy through eating, or getting sleep. In the case of women, this function is very weak compared to how it is in men, but they are able to make up for what they lack by absorbing the spirit energy released by men around them. However, in the case that there is demonic energy around them, they take in the demonic energy of monsters in place of the spirit energy that they're lacking. Because of this, their spirit energy recovery function, which was weak and lacking to begin with, completely ceases. The demonic energy settles in the woman's body and ends up causing monsterization. Additionally, when a man becomes an incubus, his spirit energy capacity is extremely increased, and his spirit energy production function also rises by leaps and bounds.
Most monsters feed on the vital "spirit energy”, that human men have a lot of. They absorb it inside their body and convert it into “demonic energy”, the vital energy of monsters. For monsters, spirit is a delicious treat that is far better than any other kind of food. Monsters that obtain and taste spirit energy will be engulfed in powerful ecstasy and joy. Even monsters that mainly eat things besides energy can still take in energy and convert it to demonic energy. To them, a man's energy is a sweet treat.
Spirit energy is plentiful in human bodily fluids, especially semen. Besides drinking it through their mouth and ingesting it, they most prefer to milk it out using their vagina by having sex with a man. Since energy is mainly taken in through sex in this manner, while “sex” is an act of reproduction for monsters, at the same time it is also “feeding.” Additionally, the taste and smell of spirit energy lingers faintly on a man's body, and is comparatively thicker on the penis. Monsters can enjoy the taste by licking a man's body, or taking it in their mouth or vagina. For monsters, a man's very existence itself is like a sweat treat because he has spirit energy within his body that they can taste and enjoy by licking, sucking, and having sex, resulting in pleasure that soothes their body and heart.
For that reason, monsters say “Delicious.” or “Looks delicious.” when they see a human man, and they refer to sex as “eating”.
Since monsters build up demonic energy by obtaining spirit, monsters that have more sex with men and get more energy poured inside them generally become stronger individuals. The more powerful a monster becomes, the more spirit energy she requires, and they never get tired of consuming it. For monsters it's like a drug: the more they absorb, the more they want, so when more spirit energy is poured inside them, not only do they become more powerful, they grow into much more lustful monsters.
The spirit energy of each individual man has a distinctive feel and taste. Humans can't sense it, but monsters are sensitive to it, and because of that, it's possible for them to sense and distinguish the presence of individual men based on the trace amounts of spirit energy that a man scatters around.
↑Early settings info; Spirits and Magic - On transformations caused by Magic
↑MGE1; Concerning Monsters: "It is also possible for monsters that use spirit as their main source of nourishment to gain nutrients from eating ordinary food, but it is comparatively far less efficient."
↑Basically the same word is used for human energy and monster energy (except when monster's refer to human energy as spirit energy) and so in the translations the term "demonic energy" is used when the context refers to the special energy of monsters. "Monster's magical energy" would also be pretty accurate.