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Light Novel / I Became the Demon Lord so I Created a Dungeon and Spend Heartwarming Time There with Non-Human Girls

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Before I realized, I had become the guardian of a dungeon, a Demon Lord. I was suddenly in a dangerous world with giant monsters loitering around, fighting for their existence, so I thought to go about developing the dungeon in order to not die but —- for some reason I was having quite the good time…. How did it turn out like this?
—Pretty much the prologue.

The protagonist, Yuki, having to live in that world now, uses his bold and sharp senses and spends time with a supreme dragon girl and a vampire girl; Sometimes laughing, sometimes being angry, passing days doing what he likes.

Includes the following tropes:

  • Accidental Marriage: Yuki makes the mistake of letting the vampire girl Illuna drink his blood, because he thinks she's half-starved and badly hurt. He did not know that this is a marriage ceremony for the vampire race. Leficious and Illuna did know, but didn't bother to tell him until later on, when he was wondering why Illuna was the only other person who could see his menu.
  • Always Chaotic Evil: The general consensus is that Demon Lords are a haughty, vicious, vile bunch that commit heinous acts for shits and giggles, and deserve extermination. All the characters of the story are awestruck, confused, and confounded because Yuki is very different, and just prefers to be left alone.
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  • Berserk Button: If you value your life, do not make threats, sexual or otherwise, against the women in Yuki's sphere of influence. Even if he's logically well aware that they can take care of themselves just fine, it will send him into a murderous rage that will have your entrails scattered to the four corners of the world.
  • Dark Is Not Evil: Demons aren't inherently evil, but they tend to be pretty aggressive and many are outright Blood Knights that pick fights with everyone around them. As such, they have a poor reputation with humans. They do get along fine with other marginalized races like beastkin, however.
  • Fantastic Arousal: Wings aren't outright an erogenous zone, but they're pretty close and one of the first features winged races like dragons look at when it comes to checking out how attractive someone is. They're also quite sensitive and touching them is a highly intimate gesture.
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  • Five More Minutes: When Illuna tries to wake up Lefi, Lefi asks to let her rest for 17 years and 24 days more, which to a dragon is probably equivalent to 5 minutes.
  • Harem Seeker: Zigzagged. The only people Yuki ever invites into his dungeon, who then become residents, are all female. On the other hand, he is clearly not looking at them with romantic interest when he does.
  • The Horseshoe Effect: In chapter 110, the king of Alyssa finds evidence that strongly indicates the Demon supremacists to the north of his nation have secretly joined hands with the human supremacist empire to the south because both sides hate the peace-loving, almost non-racist moderates that his kingdom represents.
  • Humans Are Bastards: While there are some humans who are reasonable, and just want to live their lives in peace, the vast majority are antagonists looking to either fatten their purse, or slake their lusts for power, wealth, and sex, regardless of the cost to others.
  • Living Structure Monster: "Dungeons" are living beings, and they bond symbiotically with a "Demon Lord."
  • Mistaken for Pedophile: While Yuki is something of a pedo - he's very attracted to Lefi and eventually marries her - girls as young as Illuna are outside his strike range. Yet every time he meets one, Lefi, Leila and Lyuu instantly assume the worst.
  • Mugging the Monster: Many of the antagonists who attack Yuki have a very, very poor sense of self-preservation, and really can't tell that the guy they're provoking is way above their weight class, until their entrails are scattered across the horizon.
  • New Life in Another World Bonus: The dungeon reincarnates Yuki to be its master, responsible for protecting and expanding it. To help with this, he's given an incredibly powerful body, and access to a system where he can spend "Dungeon Points" on stuff like traps, summoned monsters, new skills, weapons, potions, equipment, furniture, games, and food.
  • Protagonist-Centered Morality: What's good for Yuki and those he cares for is inherently good and wholesome, and what opposes him is inherently evil. Fortunately, Yuki's moral stance is very firmly in the Ideal Hero category.
  • Reincarnation: Yuki was an Earth human that reincarnates as a Demon Lord.
  • Slavery Is a Special Kind of Evil: Slavery is actually illegal in this new world, but it is still practiced because the aristocrats profit from it either in terms of labor, soldiers, monetarily or by perverts getting special trophies. The aristocrats notably put pressure on what passes for law enforcement to ignore the criminal rings supporting the practice.
  • Surprisingly Realistic Outcome: A giant demon castle that's impressive-looking and filled with traps? Sounds awesome... until you actually try using it as a living space. Such grandiose castles are typically made to accommodate large royal families and their armies, which Yuki and his small group are not, so his castle ends up being a titanic waste of space. Fortunately, he comes to realize that such a castle still has a use as a decoy...
  • Tastes Like Friendship: Yuki is able to "tame" the Supreme Dragon Leficious by bribing her with chocolate, which doesn't exist in this world except through Yuki's DP menu. She moves into his dungeon as a permanent resident from then on.
  • Trapped in Another World: Yuki came from Earth and is now in an alien world, with no way back. Suits him just fine.