The Demon Lord, alternatively translated Monster Lord or sometimes Maou, is the leader of all the monsters in the Monster Girl Encyclopedia world. There have been multiple Demon Lords, and the title may have been held by very many monsters since the creation of the world. Previous holders have been bloodthirsty anthrocidal killers in the classical "monster" tradition, but the current incumbent, a succubus, is rather more a lover than a fighter.
The Current Demon Lord
The current Demon Lord is responsible for transforming monsters into the human-loving creatures we know of today, and is the mother of all the lilim. Other details on her personality and interests (beyond sex) are sketchy, in spite of her central role in the Monster Girl Encyclopedia. She is at least mentioned to deeply love her husband, and the human species in general. Appearance-wise, "[lilims] have pale skin and pure white hair, which they share in common with the Demon Lord and red eyes full of demonic energy". Also, the story on the punishment of the Tentacle Forestprobably relates to an adventure she personally embarked on early in her reign (although the "white-haired succubus" to which it refers could conceivably be one of her daughters instead).
Although all monsters would agree that the Demon Lord is, at least in a nebulous sense, their "leader", the degree of control she exerts over different factions and species varies considerably. For example, while the monsters of Zipangu appreciate and respect her for giving them female forms, they are in practice almost completely autonomous. Nevertheless, the Demon Lord's metaphysical sovereignty over the monsters does affect their behaviour: just as her succubus nature influences their bodies, so too her personality influences their minds, giving all monsters a preference for monogamous relationships and not stealing other girl's husbands.
Usually, Demon Lords come to power over many years by butchering their way to the top of the pile of slavering inhuman monstrosities, with (allegedly) the tacit approval of the Chief God, having perceived that the human population needs thinning out a bit. For reasons unknown, the present Demon Lord was able to short-circuit this process, gaining the title by acclamation from the other monsters. While most monsters in former ages literally ate humans for breakfast, the Demon Lord was instead motivated by a deep love of those creatures that were supposed to be her enemy (whether this is a general trait of the succubi or an anthrophilia particular to the woman herself is not clear), and so she set in motion a plan to end their eons-long conflict forever, by transforming monsters into monster girls and trying to fuse the two species into one. Together with her human husband, she drove off the Chief God in battle and sent forth her force to her expand influence on the world.
The Demon Lord's original plan - to cast a grand sorcery that enabled humans and monsters to interbreed - was only partially successful. Although it created the monster girls, at present their coupling with humans can only produce female offspring: that is, more monster girls, whereas the plan was to have monster girl daughters and incubus sons. Obviously, the current situation represents a slow-moving demographic train crash that'll eventually lead to the extinction of monsters and humans. In order to build up magical power to push the final stage of her plan through the Chief God's metaphysical resistance, the Demon Lord spends most of her time holed up in her castle in Royal Makai screwing her husband.
The Demon Lord. The one who presently rules over the monsters is a kind of succubus who possesses extremely strong power even rivaling that of a god. She has a human (incubus) husband. She loves humans, and when she met her current husband, she started having the ambition to merge humans and monsters into one (Reference “Concerning the fundamentals of the world and the way in which it came to be.”). Her vast magical power is being used to maintain the principles of the world, and she makes great efforts day and night to further increase it. Since she's a succubus, of course the way she does that is by having sex with her husband. She deeply loves her husband, but she sure as hell tends not to show it. It is said that her minions get spooked from seeing how she fights with her husband. The world is in danger from a married couple's fight.
Originally the demon lord was merely one of the living things created by the chief god, and had nowhere near enough power for it to be possible for her to overwrite the chief god's settings. However, fortunately enough, because of several overlapping coincidences, she was able to achieve it. One was that she happened to be a succubus, a being that builds up power by having sex with men; one was that her current husband, who was the hero at the time, sympathized with her ambition and cooperated with her; one was that the monsters lent her their strength for some reason even though up to that point they had always fought endlessly amongst themselves, killing even their own kind in the struggle to decide a demon lord; and one was that just as in the case of the demon lord, a new chief god took over, but she was a goddess who was still very inexperienced. The state of the present is the product of all these coincidences.
However, the settings that had been determined by the chief god weren't so weak that she could completely overwrite them with just that. A number of problems remained that the demon lord hadn't foreseen. Although their objective changed from killing and devouring flesh and blood, to mating and milking semen, the setting “monsters assault humans” remained. Additionally, since the setting “monsters are biologically superior beings to humans,” also remained, nothing but the monster girls who are both mothers and superior beings actually ended up resulting from the coupling of humans and monster girls, even though the demon lord had expected that the incubi, who are the males of the unified race, would also have been born. Because of this, the racial unification that the demon lord seeks is impossible. Instead, the humans will probably eventually die out gradually just the way the present gods and chief god insisted it would be. And now more than ever, the chief god and demon lord's powers are locked in a bitter struggle. It wouldn't be strange if the balance crumbled at any moment. If the demon lord is defeated by the chief god and she loses her life, the monster girls will revert to being “monsters” whose heads are full of nothing but the desire to kill and eat humans. Should it come to that, the happiness of the monster girls who married and settled with their beloved human men will be shattered, they will end up having to kill and eat their own beloved husbands with their own hands.
The “Demon Lord” who is considered to be an absolute being by the monsters is an exception to the rule of vague, loose hierarchical relationships. She reigns supreme over the monsters. Fear and respect towards the Demon Lord is imprinted in their instinct. All of the monsters are subordinate to the Demon Lord, and they know from birth that the Demon Lord's energy flows within them.
In the age of the former Demon Lords, the Demon Lord ruled the monsters through terror and fear using its immense power, but, appropriately for a succubus, the current Demon Lord rules the monsters by giving them more pleasure and happiness. The current Demon Lord doesn't bind the monsters through power and laws, but the Demon Lord and the monsters are linked through demonic energy, and the monsters are strongly influenced by her, so it's possible for them to know her will to a certain degree. Formerly even the Demon Lord position was decided by monsters killing each other. New Demon Lords rose to power through rebellions, but the current Demon Lord prefers pleasure, and doesn't like killing, and because of the way she rules, the monsters of the present have no desire whatsoever to rebel.