The Demon King is a Stock Character popular in Fantasy works, especially in Japanese Media, where similarly they are usually referred to as the "Maou" (魔王 - literally "Demonic King" in Japanese). The Demon King rules over demons and often various other supernatural creatures, such as The Undead, Vampires, Werewolves and Youkai. They also are at least nominally in opposition to The Hero, who is usually literally referred to as "The Hero" / "Yuusha" (勇者 - literally "Heroic Person" in Japanese). This opposition may not even have an explanation other than "It's the Hero's job to fight the Demon King."
While the character was originally played straight as an archetypical villain, it quickly reached the deconstruction or parody phase, and more often that not, he's either an Anti-Villain who is just misunderstood, a Designated Villain who isn't even evil or shown doing anything particularly bad, or an outright Hero. In fact, it's very common for the plot to invoke the fact that he's just a Designated Villain up against a Designated Hero, and for the story to actually be about the two fighting their destinies.
Demon Kings are prone to having beautiful daughters who need protecting. Other times, the Demon King blows the hero's mind by actually being a woman. Romances between Heroes and Demon Kings are also quite common, as it turns out.
This is something of a Dead Unicorn Trope in the sense that the "Demon King" as a concept is based on a cliche in RPG Video Games where a hero fights an evil villain simply because the game tells you he's evil. It probably never really existed in the form that this trope takes, however.
Compare Demon Lords and Archdevils and Monster Lord. A subtrope of Evil Overlord. While Demon King Nobunaga can overlap, that's for a general Historical Villain Upgrade for Oda Nobunaga, it doesn't mean he's literally demonic.
- In Datte Maou-sama wa Kare ga Kirai, there are two families in present modern day, one destined to be heroes, the other Demon Kings. A Hero and Demon King are born from their respective families each generation and have the ability to attract followers. In the story, the new Demon King wants nothing to do with the family tradition and peacefully runs a shop. When the Hero (who is a lot more villainous than him, as it turns out) appears to 'defeat' him, things, er...get sexy.
- Played with in Dog Days with Valerio Calvados. The title of demon king is merely something that others gave to him due to his study of demons, a title he embraced in order to keep complaint nobles on their toes (and because it was fun). In reality, he's the former crown prince of Pastillage and the hero king that supposedly defeated him is actually one of his comrades (and is implied to be his wife).
- Dragon Ball:
- The Big Bad of the third and fourth arcs are the Demon King Piccolo and his son/reincarnation Piccolo Jr. They are a Literal Split Personality of Kami and represent his evil side, to the point where they were classed as demons.
- The Majin Buu Arc has Dabura, who serves as The Dragon to Babidi. A Big Red Devil with petrifying spit, he's stated to be the King of the Demon Realm and is as strong as previous Arc Villain Cell.
- The title of "Demon King" gets twisted around quite a bit in EDENS ZERO, where it belongs to the captain of the titular ship, with the first holder of the title being Ziggy—a kindly old robot who performed as the villainous Demon King at a theme park—before it gets passed down to his adoptive grandson, Shiki. After several Lampshade Hangings that the title sounds like it belongs to an archetypal villain, it's clarified that it actually refers to the King of Magimech (魔械 Makai, a portmanteau of the words for "magic" and "machine"), who shares his Ether with machines. However, Ziggy winds up playing the trope straight when he comes back wrong and becomes to Big Bad.
- Floor Ni Maou Ga Imasu is a workplace comedy about a demon queen and one of her minions who escaped from her world, only to end up freeloading under a restaurant.
- Hero Union BBS: The vast majority of worlds the players are stuck in have a demon lord as final boss, with variations on this archetype including having to defeat a hundred of them before finally revealing the real one, falling in love with the hero, the demon lord's son joining the party, etc.
- Koro-Sensei Quest! is an Elseworld spin-off of Assassination Classroom in which Koro-Sensei is a Demon King in a Japanese-style RPG-Mechanics Verse and his class are Wizarding School pupils.
- 3×3 Eyes ha Kaiyanwang, the lord of the Triclops, revered as the Demon King by the rest of the people of Darkness (monsters, demons, youkais you name it) who offer sacrifices to him in hope of receiving rewards. Rather than just ruling the world like his technical predecessor Benares (now his Dragon) did, he has longer term plans for the world at large, namely, collecting the primordial light from the entire humanity, merge with it and start a new journey across the cosmo in search for a new place to inhabit.
- Superior is about the demon lord name Sheila who decides to masquerade as a weak demon and follows the Hero because she's bored. She plans on killing him eventually, but finds it increasingly difficult because she's falling in love with him.
- Uchi No Maou Kamimasen Yo is a comedy about a world where people have "magical pets" that occasionally do battle, but not in any way that's taken seriously. The main character has a "Demon King", whose arch nemesis is a pet that is "The Hero" (and a girl). Although they fight instinctively, the Demon King really just wants her to like him.
- In Ramayan 3392 AD, Ravan is technically not a demon (he's actually a sentient mass of nanites), but he serves much the same purpose as a Demon King, leading the Rakhshasha hordes in their efforts to conquer Aryavarta.
- David Eddings uses the "King of Hell", who is referenced in both The Belgariad and The Elenium, series that don't by any evidence share a universe. The King of Hell seems to be a generic Satanic Archetype, but in any case is apparently bound in Hell and unable to intervene in the mortal world, and so his influence in the stories is negligible. In the later part of The Belgariad, Demon Lords that are hypothetically subservient to him are able to interfere thanks to being summoned by hapless mortals, but they primarily serve their own whims.
- In Eric, the Wizzard Rincewind finds himself going up against Astfgl, Lord of Hell - who has deliberately engineered Rincewind and Eric's adventures and finally brought them to the Discworld's Hell.
- In Journey to the West, most of Sun Wukong's opponents are called Dawang (Great Kings) or Mowang (Demon Kings), being evil demon lords ruling over hordes of minions, possessing magical weapons or skills and residing in far away, hostile caverns in the wilderness, all generally hostile to the hero or wanting to eat his protege Tripitaka.
- Satan from The Devil Is a Part-Timer! was the king of demons in Enta Isla before being defeated by Emilia the Hero, forcing him to flee. Now he attempts to reclaim his throne by working his way up the wage slave ladder at MgRonalds, going by the name of Maou. Unlike most examples, he wasn't born into a position of power, but rather started off as a lowly grunt.
- The Hero is Overpowered but Overly Cautious: The setting contains multiple worlds in need of salvation, with each one under attack by a different Demon Lord, making the title a lot less unique than in other works. Additionally, the Demon Lords worship their own evil god, who grants them power if they successfully conquer a world.
- In KonoSuba, the Big Bad is the unseen Demon King who Kazuma and his friends are working to defeat.
- Kyo Kara Maoh! has Ordinary High-School Student Shibuya Yuri, after being literally flushed to another world, crowned as the Demon King of their realm. His latent water-based powers are first harnessed when he calls out on his fiancee Wolfram (whom Yuri unknowingly got engaged due to slapping him per the Great Demon Kingdom custom) for almost killing a maid in their duel. As the series progresses, Yuri has to deal with his powers, the day-to-day matters of his kingdom and his subjects, most likely his equally handsome courtiers, and the impending wars of its neighbors such as Big Cimmaron.
- Maoyu revolves around the Demon King (who is a girl) and The Hero, who make a pact together to bring peace to the land and stop the fighting between humans and demons. It's also one of the more prominent subversions of the Maoh concept. The female Demon King is benevolent, intelligent and ultimately the most heroic character in the series. Yuusha (The Hero) might be invincible in battle, but it is Maoh's brains that hold the promise of a lasting peace.
- Kamen Rider Zi-O: Ohma Zi-O, the series' Greater-Scope Villain, is sometimes referred to as "Maoh" (particularly by his Hammy Herald Woz). Even though he's a human being rather than a literal demon, the fact that he destroyed half of humanity and rules over the shattered survivors with an iron fist more than qualifies him for the title — even moreso because 50 years prior he was a completely average, kind-hearted high schooler.
- Dracula is explicitly referred to as the "Dark Lord" in several stories, specifically translated from "Maou" in the original Japanese. Though an Expy of Dracula from the novel of the same name, his role and power are both greatly expanded. He is considered the Ultimate Evil of the Castlevania franchise and the opposite of God. No matter how many times he's killed, he returns every 100 years (minus the occasional early resurrection), and the armies of Hell (or whatever alternative name for it is used) follow his command.
- In the Sorrow series of games, Dracula is finally Killed Off for Real when his castle (the personification of his power) is sealed inside of a solar eclipse. However, the forces of Chaos (the aggregate of all human evil) reincarnate him as Soma Cruz. Soma refuses to become the new Dark Lord, but this simply means that Chaos will wait until another candidate is born that will accept the role. In the non-canon bad endings of the games, Soma does become the new Dark Lord and is an exact copy of Dracula gameplay-wise.
- In the Lords of Shadow series, Dracula's role is very similar except for the fact that Satan also exists. God, Dracula and Satan are all considered three corners of the same power balance. Yes, Dracula is just as evil as ever in this series, but so is Satan (and God, depending on how one looks at it).
- Dragon Quest VI: The first such demon is Murdaw the Dread Fiend, but the player ends up defeating another three before finally defeating their leader the Archfiend. There's another, even more powerful demon lord called Nokturnus, and if the player manages to beat him fast enough he'll take out the Archfiend for you in a Curb-Stomp Battle.
- The Legend of Zelda:
- In the series' cosmology, at least as it has been so far revealed, the progenitor of all evil is the Demon King Demise, who was destroyed in Skyward Sword, with his lingering hatred giving birth to the various nemeses Link has faced in the other games. He also has subservient demon lords, most prominently his dragon, Ghirahim.
- The successor to Demise is Ganondorf. While he starts off as a mortal human (a Gerudo to be precise), his quest for power eventually transforms him into a pig demon known as Ganon. Ganon, like Demise, exhibits an evil aura and rules over all the monsters that Link faces in his quest. Games released after Skyward Sword would later refer to Ganon as a Demon King rather than just King of Evil or Dark Lord to signify his status as Demise's reincarnation.
- Malladus from Spirit Tracks is often referred as the Demon King. He once terrorized the land that would become known as New Hyrule before the Spirits of Good imprisoned him with the spirit tracks. He has a subservient demon known as Chancellor Cole who seeks to resurrect his master with Zelda's body, as well as an fleet of demonic trains to do his bidding. Interestingly, he's the first villain to bear the title "Demon King" in the English version, before Demise and his connection to Ganondorf were revealed, and Spirit Tracks took place long after The Wind Waker, implying that Malladus became Demise's new incarnation of hatred once Ganondorf was Killed Off for Real in that game.
- Ghaleon from Lunar: The Silver Star belongs to a demonic tribe, starts out a hero, then becomes a demon lord (albeit a Well-Intentioned Extremist)... Then in the sequel redeems himself.
- The Nippon Ichi multiverse is full of Overlords, and most of them are demons. A good chunk of the games either have you playing as one or trying to defeat one.
- Bowser from Super Mario Bros. is known as Daimaō Kuppa in Japan, which means "Great Demon King Koopa". He's the king of the Koopas, a tribe of turtles who are inspired by the yōkai demon known as the Kappa. And his Koopa Troop army has other monsters like the Goombas, Boos, and Piranha Plants to name a few. He also has a son named Bowser Jr. and wants to marry Princess Peach of the Mushroom Kingdom.
- Dot Kareshi features JRPG stereotypes as love interests and naturally, the third game focusing on villains has a Demon Lord (though he also has traits of the more modern white haired pretty boy JRPG final boss trope), who developed an inferiority complex after the player steamrolled him after Level Grinding.
- Forbesii is the king of the demon people in SHUFFLE!. He's a very chill guy, best friends with his godly counterpart Eustoma, and his beautiful daughter Nerine is one of the game's main heroines. Forbesii also plays a bigger role in the sequel/spin-off Tick! Tack!