Demonic energy, often abbreviated DE and alternatively translated as monster's mana, is a kind of "life force" which is possessed by monsters in the Monster Girl Encyclopedia world. The yin-yang analogue of the spirit energy which is possessed by humans and other mortal creatures, demonic energy sustains monsters in a basic, metaphysical sense, as well as powering their magical abilities. It has several powers that spirit energy does not; most obviously its ability to corrupt creatures, environments, and even gods into demonic perversions. However, its disadvantage is that demonic beings do not generate this energy within themselves like spirit beings do. Instead, they must consume spirit energy in order to convert it into demonic energy. This is one of the reasons that monster girls hunger insatiably for semen: it contains more spirit energy than anything else, so constitutes a feast for their demon constitutions.
When highly concentrated, demonic energy can appear as a physical substance: a black, tarry goo that spontaneously forms into ominous, tendril-trailing spheres. Too much in one place may spawn its own elemental, a dark matter.
Technically each species of monster has its own demonic energy: so there's werewolf demonic energy; bubble slime demonic energy; vampire demonic energy, etc. In the age of the former Demon Lord, these energies remained with their respective species, but the current Demon Lord has forced her own succubus demonic energy into every species. This is why the monsters are now monster girls, with curvy female bodies, sky-high lusts, and the ability (or requirement) to use semen as a dietary staple. It's also why getting bitten by a large mouse turns female victims into the same monster: when enough of the relevant type of demonic energy enters her body, a woman transforms according to the specific source of her magical corruption.
As well as powering monsters' basic existence in general, demonic energy in surplus strengthens their spells, boosts their physical abilities, ramps up their lusts, and lengthens their lifespans. Different monster species innately have different amounts of this energy, which is what accounts for the power difference between, for example, slimes and lilims. Nevertheless, all monsters from slimes to lilims can massively boost their powers by amassing more demonic energy; and the way to do this is to have more sex and take in more spirit energy. As with humans and elves and dwarves, monsters' exertion, spell-casting, and just generally being alive causes demonic energy to be lost from their bodies. Although they can survive on "starvation rations" with the spirit energy contained in normal foods, this is both unpleasant and unhealthy in the long term.
In spite of the monster girl's general hedonism and indolence, demonic energy spellcasting seems to be one field where monsters are better developed and researched than their human nemeses. The initial remit - and, indeed, the official name - of the Sabbath HQ is the "Demon Lord's Army: Spell Casting Unit", and despite veering the organization at right angles to become a loli supremacist religion, the baphomets continue R&D of demonic energy spells. Individual liches often take up demonic spell research too.
The passive effects of demonic energy are possibly more important than the active, hex-throwing ones: it transforms human lands into ominous demon realms, and then induces mana corruption in females who enter them. On a more personal level, the part-succubus nature of the energy causes mental, physical, and metaphysical aspects to turn towards sex. Infirmities heal, moods turn lustful, and a crazed ushi-oni with knives for feet and a grip that crushes boulders can savagely rape a petite shota for days on end without anyone dying.
Demonic energy is partly empathic in its effects. For married monster-incubus couples, the magical effect subtly molds the bodies of both towards their opposite number's preferred sex-fantasy: most obviously, incubi changing their age-appearance to match their mate's taste . To an extent, demonic energy is also shaped by the Demon Lord; monsters' usual preference for monogamy, and not stealing other monster's husbands, is attributed to some of her personality traits being transmitted to her subordinates  via demonic energy.
Demonic Energy is different from the human perception of it (ie, it's really DE, not Spirit), and is inversely proportionate to Spiritual energy. However, because they're both commonly known as MAGICAL energy, there's not much of a distinction.
Demonic Energy (Henceforth referred to as DE) can be said to be the life-force within all monsters.
Although DE can usually be produced by the consumption of food, monsters like succubi require human Spirit Energy for DE production. in their bodies. Succubi in general are better able to channel DE as opposed to Spirit Energy, and project it out of their bodies. Human males bathed in this energy will find their lifespan lengthened and their sexual desires increased.
Due to the high concentration of DE that a succubus possesses, human males involved with the succubus often will have DE injected and spread deep into their bodies, allowing them to store both DE and Spirit within them. This results in a transformation in personality, and they become known as Incubi (Single: Incubus).
It is also worth noting that the realm full of (and transformed by) this Demonic Energy is appropriately referred to as the Demon Realm.
The demonic energy of monsters is a vital energy that all monsters have inside their bodies. It's relationship with the previously discussed spirit energy is like the other side of the same coin. Since the spells they use also consume this energy, both it and spirit energy are generally referred to as “magical energy,” and there isn't a great distinction drawn between them. Demonic energy is a general term for the magical energy that all monsters possess. Every race has its own particular kind of magical energy. Technically, if it's a succubus, then it's called succubus' demonic energy, and if it's a werewolf, then it's called werewolf's demonic energy.
“Demonic energy” is the vital energy that all monsters have inside of their bodies. Monsters always have a race specific “demonic energy” along with “succubus' demonic energy” that pours into them from the Demon Lord. Compared to humans, many of them have high magic power. Since demonic energy is mainly produced by absorbing “male human spirit energy” and converting it into demonic energy inside of their body, it is purported that the more an individual has sex with men, the higher the magical energy she will possess.
Due to this magical energy, the spells they use are often vastly more powerful than similar spells used by ordinary humans. Some of them even have magical energy powerful enough to level a small town. However, of course, being monsters, they wouldn't use their magic power for something like that. Instead, it is mainly used for spells to seduce men, or enhance sex with men. “succubus' demonic energy” is an energy that is especially well suited for charm and aphrodisiac spells, etc. In the present, since all monsters have at least a bit of succubus' demonic energy, the charm spells which were only used by some monsters such as succubus, etc. are now regarded as the chief fundamental spells that monsters use. Furthermore, not all monsters can use high level spells. Even if all monsters have magical energy inside their bodies, they may not know how to use that energy freely, or they may not have the intelligence necessary to use it. Some races are not good at handling magical energy, and some can't handle it at all. For such reasons, there are many monsters that can't use spells at all.
Aside from spell casting, small amounts of demonic energy are naturally continually released from the monsters' bodies. Monsters are always in a state where they're draped in demonic energy. This demonic energy that is released is used by them unconsciously for various purposes. When they have sex with a man, the energy serves to protect the man's body. No matter how roughly and how long they have sex, the man's body never enters an abnormal state worse than fatigue, and he can never die due to sex. It also ensures that the man won't be harmed by the monster's horns, scales, etc. If the man touches the tip of a pointed horn or scale, the demonic energy covering the horn or scale gently pushes him back with a feeling like the touch of soft skin.
This is because the current Demon Lord belongs to the succubus family. Presently, the succubi are the core of the monsters. All monsters that are linked to the Demon Lord via demonic energy possess “succubus' energy” in addition to their own race's energy.
In the case where the demonic energy of several races is mixed together, the succubus' energy takes precedence. The demonic energy of other races is all taken in and converted into “succubus' energy”. For that reason, even though the demon realm is full of the demonic energy of a diverse variety of monsters, it's still all basically that of the succubus. When a human woman is violated by the demonic energy of the demonic realm, or eats a fruit that was raised in the demon realm, she'll naturally become a “succubus.”
Additionally, if there isn't a diverse variety of monsters, and instead it's a concentration of just the same race, and the demon realm is full of just that race's demonic energy, or, the overwhelming majority of the pure demonic energy belongs to that race, in that case a human woman would transform not into a succubus, but into that race instead. In other words, if there were a demon realm where only slimes and nothing but slimes lived, being violated by the demonic energy there, or eating the food grown there would cause a woman to transform into a “slime” instead of a “succubus”.
↑MGE-1; Mental and Physical Changes Caused by Incubization: "Just like monsters, their appearance stops aging at a certain stage. It could be as a boy, as a young man, or as a mature man. Since an incubus is a being that's convenient for a monster, they change depending on the tastes of the monster that they're paired with."
↑MGE-1; Relationship with Human Men: "it may also simply be explained that the personality of the current demon lord who changed their traits is showing up in the instincts of the monsters."