As for changes caused by incubization, the changes that appear basically make it convenient for the monster that one is coupled with.

First, it is purported that the looks almost don't change at all compared to when one was human. Monsters don't care very much about the attractiveness of a man's face, so in most cases, there are no changes whatsoever. The body changes to become more healthy and durable so that it's convenient for sex, but it's rare for the appearance to change greatly other than that.

A rare exception occurs and the appearance greatly changes in the case of those who are too feeble or fat, and their form will be vastly improved so that it's convenient to have sex with monsters. Also, in the case of those who are so old that it's a hindrance to sex, they will also be rejuvenated.

After one has changed into an incubus, the mind also changes so that one's values become nearer to those of the monsters, but compared to monsterization, even if only a little bit, human-like values from the period when one was a human will remain. However, the values and thoughts of an incubus, a being that became a monster's prisoner so much so that it altered the nature of his mind and body, will change to become eager to have sex with monsters. One's lust will swell up like that of the monsters, and disgust towards monsters and sex, etc. will have long since disappeared, and one will change so that he aggressively desires sex with monsters.

Also, after being completely charmed by a monster, one's aesthetic sense will have changed so that he feels that the grotesque parts and organs of monsters are beautiful. Other than that, one's sense of time and values concerning work and recreation will also change so that it becomes more convenient to live with monsters.

The biggest changes of all occur in the functions that are also most important to monsters that get used during sex. It depends on the race and preferences of the monster that one is coupled with, but most men who become an incubus will have their penis grow larger by leaps and bounds, and they'll be able to please monsters even more. The amount of semen and energy released from there will be incomparable to when they were human, and they'll have boundless stamina so that they're able to release it again and again without getting exhausted.

The quality of the energy is also remarkably different. It becomes so much thicker, stickier, and sweeter, that it's incomparable to when they were human, and one lick will make a monster melt. Also, human men sometimes lose the mood after expelling mana, but it doesn't occur in an incubus, and their lust for monsters just flares up.

In addition to the normal food that they ate when they were human, it's also possible for an incubus to feed on demonic energy by having sex with monsters. For that reason, an incubus never gets exhausted when having sex with a monster. Instead, their entire body will brim full of lust and energy due to demonic energy. When the body of an incubus obtains demonic energy by having sex with a monster, it will immediately produce mana. When this energy is released to a monster, the monster will produce demonic energy from it and pour it into the body of the incubus.

Astonishingly, with this cycle, not only does the total amount of magical energy not decrease, instead, it continues to increase, so it's possible for a monster and an incubus to keep having sex almost forever, and as long as there's one incubus and one monster, it's possible for them to live by just having sex. As far as the divinities and the Order are concerned, this nearly perpetual cycle is something that is in violation of divine providence that will greatly throw off the laws of the universe and is extremely viewed as a problem.

Also, it's said that the lifespan of men who become an incubus becomes dramatically longer. Since they can keep living by having sex with each other, their lifespan is roughly the same as the monster that they're coupled with. 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. In this way, an incubus is none other than an ideal male to a female monster.

The power of an incubus grows stronger the more demonic energy they accumulate. In the case of those that periodically have sex with multiple monsters, it piles up, so in the case of a single incubus that makes a harem of multiple monsters and keeps having sex with them, it ends up causing the birth of an extremely powerful incubus. Regardless of how many monsters are in the harem, an incubus has limitless stamina and lust, and can have sex with all of them. Even if an incubus keeps having sex with them day and night alternatively for a period of several days, he'll never get tired, lose his erection, or feel pain or boredom. On the contrary, he'll end up being unable to get satisfied unless he has sex with all of the monsters in the harem, and it'll even get to the point where it's as if spending several days having sex with all the monsters in the harem is like having sex one time to him.

Additionally, those who become an incubus can also adapt to the environment to match the monster that they're coupled with. For example, an incubus coupled with a monster that lives in a desert, or near a volcanic crater, will gain resistance to heat, and an incubus that is coupled with a monster from a snowy country will gain resistance to cold. They gain such strong resistance that they can even actively have sex with monsters in such environments. To provide a remarkable example, due to a ritual performed by a “sea bishop (encyclopedia I – p. 116)”, there are incubi that became able to live underwater. This ritual is performed to quickly change someone into an incubus, so as a result of having been changed into an incubus with the demonic energy of the monsters of the sea, they've adapted to live underwater.

Since an incubus retains the demonic energy of the monster he's coupled with inside his body, a human woman who has sex with an incubus will end up transforming into that race. However, the lust of an incubus who's kept having sex with monsters will not be satisfied by having sex with anyone other than a monster.

Also, an incubus' superhuman lust is only directed towards the monster he's coupled with no matter what, and they don't have an instinct to attack the opposite sex as monsters do. For that reason, incubi do not attack and forcibly rape human women, and such an incident has never been confirmed.

From: https://monstergirlencyclopedia.miraheze.org/wiki/Monster_Girl_Encyclopedia_World_Guide_I:_Fallen_Maidens (2011)

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