Incubus

An incubus is the male counterpart to a succubus, and represents the fate of (most) males exposed to high levels of mamono mana. Unlike female monsterisation, where the physical and mental change from human to monster is rapid (in some cases taking only moments ), for incubi the process is much more gradual and open-ended. Just as monster girls become more and more powerful the more sex they have, so do incubi, with increases to lustiness, physical attributes and lifespan.

Incubi are said to be "an ideal male to a female monster", therefore many monsters are eager for their mates to transform faster so they can enjoy the changes. Monsters with a lot of mamono mana at their disposal, like dark matters or khepri, are noted as being able to effect the change quickly, but the majority of monsters must just rely on lots and lots and lots of sex. Certain items which can expedite the transformation, like Succubus Nostrum, may be sought by eager wives.

Although the vast majority of information on incubi pertains to human men, elves can definitely become incubi and there seems no reason that any of the few male dwarves left can't do so as well (or rather that they didn't already become incubi some time ago), as dwarf men are also considered a type of human.

A Being That's Convenient For Monsters

 * Refer to: Fallen Maidens; The Incubus

Physical changes
Besides genital gargantuism, incubi do not receive (permanent) drastic physical changes comparable to, say, an arachne's abdomen or a succubus' wings and tail, although their eyes may change color due to mamono mana (the source for the eye color change is not canon). They do, however, see disposal of features that impeded their sex lives previously, such as corpulence or the general frailties of old age. And as the time spent with their monster partner increases, incubi find their appearance and even their apparent age adjusting to match their wife's taste: "It could be as a boy, as a young man, or as a mature man". Since this happens simultaneously with the man's tastes reshaping her, eventually both monster and incubus will gain their spouse's ideal fantasy body.

There are changes under the skin for an incubus too. Stamina increases to monster-like levels as the eldritch power of mamono mana suffuses their body, and incubi gain some measure of the environmental resilience of their monster partner (e.g. heat resistance for a lava golem's mate, breathing underwater for a Sea Bishop's mate, night vision for the mates of nocturnal or cave-dwelling monsters ). Like a succubus, they also gain the ability to actually sustain themselves on their partner's mamono mana in place of food.

An exception to the lack of drastic changes are the husbands of the various chaos/abyss-aligned monsters (such as atlach-nacha and mindflayer), which gain the ability to change into completely non-human or only vaguely humanoid forms. These forms can be reverted to normal human form at will, at least after the initial forced transformation, and thus are not considered a permanent transformation.

Mental changes
On first being attacked by a monster girl, humans are likely to be consumed by an instinctive fear and revulsion at their demonic appearance. Such atavistic terror and Order moral objections are thrown aside with the transition to an incubus: Their thoughts and values change, all becoming centered around having sex with monsters. Lust skyrockets to keep pace with the monsters' insatiable hunger for sex, and their interests in other things (like work and hobbies) can potentially change, as sexual activity becomes a more important part of their lives. There is a lot of variety though depending on the inner nature of the man, some incubi lead completely "normal" lives during the day, restricting sex to night and having it replace most of their sleep as it is just as energizing for them:

Incubi never get bored of sex or lose the mood after doing the deed; they can keep going indefinitely thanks to exchanging energy with their mate in a constant cycle. Whereas a human first being attacked by a monster is likely to pass out from pleasure before his monster girl partner is close to being finished with him, an incubus can go until perfect mutual satisfaction is reached, and incubi with harems can flip the dynamic on its head and "end up being unable to be satisfied, unless he has sex with all of the monsters in his harem".

Magical changes
When women undergo monsterisation, their essence production ability (which is weaker than men's to start with) gets extinguished entirely and they become a creature with only mamono mana inside them. The same does not occur with men and incubi, instead there is a large improvement in mana production in response to increasing amounts of mamono mana within their bodies.

The mamono mana that enters a man, "marks" an incubus with his partner's uniquely-scented mana. There is a general understanding that other monsters will not randomly ravish a man who has been claimed like this, albeit should another monster fall in love with him then the marking will not be a barrier to her love and a harem will form instead. Although human women can't detect it, the mamono mana can have a drastic effect: if an incubus has sex with a human woman, his semen will monsterise her. Notably though, no incubus is known to have ever had sex with an unwilling woman, and excepting if they already were in love with a human woman, their lust naturally is directed toward monster girls anyway.

Daily lives
Although the sex drive of a man who has become an incubus is greatly enhanced, causing sex with their monster partner or partners to be a greater part of their life, incubised men are still free to pursue previous hobbies and work from before incubisation. This often results in monster nations exporting produce to other nations, sometimes producing goods that can only be created by a monster and her husband, or are only available in mamono realm lands.

A large cultural shift has begun amongst the monsters more recently, with monster wives taking a keener interest in more human activities, whether it be culinary or a craft. Many of these changes have occurred as monster women have sought to improve the lives of their husbands, helping to keep them happy. Many single monster women have also taken up crafts to further impress potential mates, instead of directly assaulting them.

True Nature
In World Guide 3, the baphomet Runya Runya shares convincing arguments that incubi and heroes are essentially the same type of being. That is, they are humans that have broken the "shackles" that are thought of as human limits, the only difference ultimately being that where heroes have their shackles broken by divine power (or sometimes other sources of energy), incubi utilize mamono mana to remove their limiters. The powers and/or abilities they gain after breaking these shackles (aside from spells and abilities specifically dependent on gods or monsters) are their own, and remain even if (for instance) all divine power is removed from them. Heretical theologians theorize that humans were created as beings with unlimited potential (that was for unknown reasons and by unknown parties later shackled), and that rather than becoming a new type of being, heroes and incubi are merely regaining the abilities they were meant to have all along.

Further evidence for the lack of meaningful distinction between heroes and incubi can be seen in how most every ability (including sexual ones) gained by incubi can potentially be gained by divinely blessed heroes and vice versa. Notably, humans blessed by Eros to become her version of "heroes" gain (among other things) sexual abilities on par with incubi.

Research into all this, however, is forbidden by the Order, and thus monsters and monster-allied humans seem to be the only ones studying just what heroes and incubi really are.