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. Monsters also innately possess, and can store, far larger amounts of energy than humans innately can.
However, its disadvantage is that they release more of the energy from their bodies than humans do, and while this does serve various purposes, it also leads to a lot of energy loss (monsters can be said to be high performance, low efficiency beings, a trait common in the higher beings of the MGE world such as gods). This loss is compounded by how, like human women, the monster's ability to recover their energy via internal generation is fairly weak. In the previous era if a monster's internal reserves were depleted too much, and there was no proper source of ambient energy to absorb (or many humans to consume) they would have to enter a period of prolonged dormancy until their internal energy generation completed its work. This comparative lack of generative ability 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 and enables rapid recovery or even increase to their max demonic energy levels.
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 (though the amount required for this is extreme, hence why there are very few 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. However, they also all contain part of the energy of whatever Demon Lord is currently in power, and are thus linked to this lord on a spiritual level. In the age of the former Demon Lord, this link was used by the Demon Lord to help organize and dominate the largely savage monster hordes, but the current Demon Lord used the link (and other special circumstances) to have her succubus demonic energy alter the bodies of 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 Created Via Intimacy Edit
Perhaps the most important of all its effects however, is that when the energy interacts with other types of mana, in certain circumstances (namely during sex or other intimate behavior) it doesn't simply consume it and gain whatever amount of energy it takes in, rather there is a reaction where after it is consumed there is more total mana than existed previously (sometimes as much as double the initial amount). This physics-defying ability was not always a part of demonic energy, and only came about when the current Mamono Lord came to power and managed to turn all the monsters into monster girls. It is as yet unknown by even the most acclaimed researchers in Sabbath as to how exactly she did it or how it works, but the result has been a major key to human and monster coexistence as well as monster prosperity.
Namely, this effect allows a monster to feed as much as she wants on her mate's energy without draining him to death (as was a common risk in the previous era) as she can freely return to him the excess energy created by her energy interacting with his, so he never truly runs out to the point of death (though he still may feel overwhelmed by the pleasure to the point of hallucinating that he's been drained dry). In a worst-case scenario it even allows for a couple to not need any outside food or energy at all, being able to subsist entirely on the energy they generate during sex. The excess energy created from sex also allows for demonic energy to be gathered and sold as a power source or for experiments without any detriment to who it is gathered from.
It is notable that this reaction between demonic energy and other types of mana is not without a requirement if one is to take full advantage of it. Namely the interaction must include genuine affection from the monster whose energy is reacting with the other mana, or the mana-creating effect will greatly weaken. If she did not care at all for her partner for instance, and simply milked him mechanically, then the total amount of mana would only increase slightly, and the monster would have to return most of what she milked in order to keep the man healthy.
- There are very many sections in the various Monster Girl Encyclopedias that cover demonic energy. In addition to the texts below, see also The Source of Demonic Energy, The Various Roles of Demonic Energy, Succubus' Demonic Energy, Mana Corruption
- ↑ 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."