Dullahan are undead knight monster girls which form the vanguard of the Demon Army. They're distinguished by their unusual seriousness; unlike the vast majority of the army's monsters, dullahan actually have some military discipline, and don't just abandon the fight as soon as they locate a rape-able male. This characteristic makes them critical to the defense of the Demon Realm from Order attacks. They were also called upon by the Demon Lord in her subjugation of the Tentacle Forest. If you are able to knock off their heads, however, their soldier-like discipline vanishes and their pent-up lust will be released.

Human women can be monsterized into dullahan through two special ways. Cursed armours can accomplish the transformation within only seconds of putting it on. Also, the Undead Grey Lamentation Mushrooms can bring about death and subsequent unlife as many different undead-type monstergirls, dullahan included.

Encyclopedia Entry

A demonic knight with a grotesque figure, its head has been severed from its body. It inhabits the Demon Realm.  Because of its identifying feature, it’s also known as the “headless knight.” They are the prized advance-guard of the Demon Lord's knights. They are extremely loyal, and possess exceptional military prowess. In the illustration on the left, they hold their head with their left arm, but most of the times they appear, they put their head on their body, so it’s often hard to even realize that they’re Dullahan.

Men’s spirit energy is what fuels them, but they don’t aggressively attack. They are very intellectual. They store all the spirit energy they’ve collected inside their bodies, and use their head as a lid, so they can get by normally even with a very low amount of spirit energy.  However, being Dullahan, their heads come off very easily. When this happens, their pent-up desire, emotion, and true intentions are spilled out along with all the energy. 

Because of this, when their head is in place, they are stubborn and intellectual, but once their head comes off, they change completely. Their personality reverts into a naturally-lustful monster personality and they act according to their true feelings, and fawn on the man to entice him in pursuit of sex. Normally they hide that personality, but it is what they truly feel on the inside and it is their true character. If one ever encounters a dullahan, be sure to take advantage. Knock off her head and you can end the fight. However, the spirit energy will spill out, and she’ll become hungry. As noted above, her personality will change and she’ll probably assault you right then and there.

And another thing, they sometimes appear from out of nowhere and come to kidnap a man they're fond of. On this occasion, they specify the date and time of kidnapping for some reason. As for those who were able to escape their grasp, it is said that there is not a single person in existence.

They form the core of the Demon Lord's army, but most of the others besides them prioritize their own desire above their duty as a knight, so their priority is to get a man and have sex. Rare among them, the Dullahans are loyal and take their duty seriously. The anxiety they experience while serving among the knights is not difficult to imagine.

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