The Monster Lord’s Castle (魔王城 Maou jou) is the final location in Chapter 2 of Monster Girl Quest, located at the northernmost section of Hellgondo. This is where Luka finally faces and defeats the Four Heavenly Knights, as well as his companion throughout his journey, the current Monster Lord Alipheese XVI.
After Alice teaches Luka Quadruple Giga, the two set off for the Castle. Luka remarks that the castle is surprisingly pretty, and in reply Alice asks who would want to live in a foul, evil-looking place. The two part ways, with Alice admonishing Luka to fulfill a hero’s duty, however painful it may be.
A little later, Luka senses a horrible presence approaching; he steels himself, and out of the darkness comes… a lowly Imp, who wasn’t expecting him to show up at all. Luka tells her that he only attacks those who harass humans, which the Imp admits she’s guilty of.
Realizing that there are a lot of ordinary civilian-type monsters in the castle, he resolves to get this over with quickly before he troubles any other innocents. After defeating the Imp, the true source of the horrible presence appears: the Giganto Weapon, a biological weapon from the war between monsters and angels a thousand years earlier. Luka damages Giganto Weapon to the point that it uses its auto-repair ability; Salamander chimes in and teaches Luka to use Vaporizing Rebellion Sword, allowing him to finish off the bioweapon. Luka then thinks about the similarity between this and the Chimera he’s already fought; why are they so similar?
Finally, he approaches the Four Heavenly Knights. They all greet him with varying levels of friendliness, and when he asks them to step down for the final time, they refuse, and Luka must convince them with his strength. He defeats Alma Elma, Tamamo, Erubetie, and Granberia. They all give their pledge to coexistence (grudgingly in Erubetie’s case), finally completing Luka’s goal once and for all.
However, Alice’s voice rings out, congratulating the hero on his progress and inviting him to face her. Dumbfounded, Luka runs in to confront her in the next room, ignoring Tamamo’s warning. Alice greets him as the 16th Monster Lord as if they’ve never met. Luka thinks it’s a joke until Alice reminds him that it is his duty to slay the Monster Lord. He realizes that she’s trying to sacrifice herself for peace, just as her mother did. Alice orders him to take up his sword and attacks. After being reduced to a sliver of health, she asks him to finish the job.
If Luka kills her, Goddess Ilias appears to congratulate the Hero She tells him to claim the treasure chest within the room, as it is his right. Inside, he finds the ribbon he gave to Alice, and Ilias then takes him up to heaven to rape him for all eternity preventing the player from unlocking part 3.
If Luka spares her, he explains that killing the Monster Lord would be against his ideals as he wants to save lives, not take them. He says that if she died, an age of peace will not come; rather, humanity will oppress the monsters in a frightening turn of events. Luka likens this to their experience in Plansect Village. Alice, however, wants to make sure her mother didn’t die in vain (“But if I don’t fall here… my…my mother would truly have died for nothing…”), to which Luka replies that her mother’s method of achieving peace was wrong—a tiny, infant Alice easily shattered the fragile illusion of peace that her mother had created. Luka then promises that they’ll travel the world together again when everlasting peace is at last achieved.
However, Ilias descends, disappointed. Luka tries to reason with her, eventually turning against her as she is the one who is preventing coexistence. Avoiding divine punishment, she announces that the castle will be a “Second Remina”, and disappears.
Erubetie comes in and alerts them that angels and chimeras have begun to lay siege to the castle; the two run out and confront them. A Vampire and a Behemoth are barely holding out against multiple Chimera Beasts, Cupid and Valkyrie. The angel Promestein appears, revealing herself to be the mastermind behind the chimeras’ creation and the leader of this attack. Moreover, the Next Dolls—artificial monsters designed to counter the Four Heavenly Knights—appear and demonstrate their power by defeating two of the Knights. Luka tries to attack Promestein, but she avoids it with Zylphe and reveals that she's studied and mass produced artificial spirits. Alice tells him that he isn’t alone in this fight, and together they charge into the fray.