The Demon King is the King of Demons, and a stock character in fantasy Japanese Media, where this type of character may be referred to as a "Maou". The Demon King rules the demon world and all the various supernatural creatures, including such things as vampires and youkai, and is in opposition to The Hero or Heroes. In this case, The Hero is usually literally referred to as 'The Hero' or the "Yuusha". Often times this opposition doesn't even have an explanation other than: "It's the hero's job to fight the Demon King, so he does."
While the character was originally played straight as an archetypical villain, it's now reached the deconstruction or parody phase and more often then 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 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, Monster Lord. A subtrope of Evil Overlord. While Demon King Nobunaga can overlaps, that's for a general Historical Villain Upgrade for Oda Nobunaga, it doesn't mean he's literally demonic.
Examples:
- 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. This is all retconned in Dragon Ball Z, which reveals that both Piccolo and Kami are aliens from another planet with no special relationship to the spiritual world.
- 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.
- 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.
- 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.
- 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. He started off as a lowly grunt.
- 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.
- 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.
- In the cosmology of The Legend of Zelda series, 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 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.
