According to the Jesuit Father Luís Fróis, Oda Nobunaga called himself "Demon King of the Sixth Heaven" (dairokuten-maō), a title properly belonging to Māra, the Buddhist counterpart of Satan (though portrayed in mythology as a Noble Demon). While Nobunaga was most likely being sarcastic, and in any case he's not the only daimyō of the Sengoku Jidai whose ruthlessness has inspired a Historical Villain Upgrade, many popular depictions of Nobunaga literally demonize him, or at least give him supernatural powers.
The biggest inspiration of this usually comes from one of Nobunaga's biggest Shoot the Dog (or most commonly believed as a kick) moment: the burning of Mt. Hiei, taking out the Buddhist warrior-monks of Ikko-Ikki and leaving none alive, including women and children. Whoever had the higher moral ground was unclear at that point (whether Nobunaga was just being callous, or the monks themselves were really disruptive and just adding on chaos in Japan that needed to be quelled no matter what, even if they used religion as a way to excuse their behavior), but considering Buddhism eventually recovered and once again became one of the major religions in Japan, Nobunaga's actions towards those Buddhists were not going to win him many fans from the future Buddhist storytellers, so they tend to use his previous sarcasm and make it a literal statement. And in addition to his more accepting stance with Christian missionaries, being considered 'barbaric foreigners with foreign religion' (more below), Nobunaga himself has been said to be rather disrespecting towards Buddhism in general, so that's even more reason to make him a devil figure by the Buddhist majority.
This is also not helped with Nobunaga's attitude during his time, while he was ruthless, there may be other factors on how Nobunaga was singled out to be the Demon King candidate instead of any other ruthless daimyos out there. In those times, traditional and old fashioned honor was especially revered. Nobunaga mostly threw a middle finger on those, as he collaborated with 'barbarous' foreigners to bolster his forces (and also wore European clothes a lot), promoted meritocracy instead of judging people based on their status (which was the norm back then) and several of his enemies that were mostly traditionalist eventually got trounced by himnote . These days, Nobunaga's actions might have been thought to be a stroke of genius, but in the days past, this was considered rather blasphemous and extremely dangerous, such that people thought only demons would think of this kind of trouncing of what was good in their eyes. And considering that even in modern Japan, traditions are well preserved and those who do not try to blend in with others and try to stick out often get ostracized, it's more than likely that Nobunaga would be seen as a violator of Japanese ways, making him perfect to be referred as an evil, tradition-wrecker demon (even if those who values individuality would see Nobunaga as a genius instead).
In the old days, this trope was used for a more common Black-and-White Morality, since Nobunaga's brutality was perfect to play the role of villain. As time passed, however, values started to shift and people learned about the grayness of war, that the Buddhists that relegated the role to Nobunaga might not have the absolute moral high ground. Because of this, the trope started to evolve. Rather than playing the Demon King factor of Nobunaga totally straight, there are also works that use the Noble Demon or Dark Is Not Evil treatment for our resident Japanese Demon King; just because Nobunaga has demonic traits or can be rather brutal doesn't mean that he always had to play the cackling Saturday morning cartoon villain.
A subtrope of Devil Complex and Historical Villain Upgrade. Compare Beethoven Was an Alien Spy, We Didn't Start the Führer and Dracula (as Vlad Tepes); another East Asian example is Cao Cao's transformation from his historical persona to stock Card-Carrying Villain in Peking Opera (though not to an extent of being a demon).
- In Black Lion, Nobunaga is actually possessed by alien invaders who equip his armies with high-tech armaments so he can conquer Japan as a beachhead (probably; the backstory isn't covered much).
- In Samurai Deeper Kyo, Nobunaga was Japan's fiercest warlord who has a massive hate on for the main character. True to his nickname, his powers all revolve around Grim Reaper based attacks. Shou Hayami voices him here.
- In Wrath of the Ninja, Nobunaga appears to be the chief antagonist for the 3 heroes, conquering Japan with demonic help and seeking to be transformed into a demon himself. This time, however, there's a Man Behind the Man, a demon looking to use the bloodshed of war and Nobunaga's cruelty to power the demons themselves, and Nobunaga is just his patsy.
- In Blood Reign: Curse of the Yoma, Nobunaga doesn't directly appear, but the story takes place at the height of his conquests, and he influences the story since the main character is a ninja serving the Takeda clan when it opposes Nobunaga, and because the brutality of Nobunaga's conquests is giving power to the Yoma demons. At one point the main character thinks about Nobunaga's brutality and wonders if he is a demon. While he's doing this, Nobunaga and his army are seen in a montage, and all of them have glowing red eyes.
- In Ghost Sweeper Mikami, the vampire lord Nosferatu is actually Oda Nobunaga. Or if Father Karasu's theory is correct, Nosferatu murdered a young Nobunaga and stole his identity. Either way, Akechi Mitsuhide eventually discovered his master's demonic nature and that was the reason for his betrayal at Honno-ji.
- In the second Peacock King OVA, Castle of Illusion, a resurrected Nobunaga rebuilds Azuchi Castle as a fortress of evil and commands an unholy army of Cherubim.
- While Oda Nobunaga himself does not appear in Batman Ninja, The Joker (time-displaced to Sengoku Period Japan, along with a few other villains and heroes) takes the title Dairokuten-maō for himself.
- In Kamen Rider OOO's movie, the Kougami Foundation created a Homunculus copy of Nobunaga; since he was made from the same materials as the villainous Greeed, he ends up becoming a similar creature. However, this is revealed to be involuntary, instead acting as a Super-Powered Evil Side who comes forth to seek revenge on the descendants of those who killed the original Nobunaga.
- The Call of Cthulhu supplement Secrets of Japan reveals that Nobunaga is but one of Nyarlathotep's thousand masks, and that the Outer God still occasionally pulls him out for, of all things, business meetings.
- GURPS Whos Who 2 has an entry for Nobunaga which discusses the possibility of playing him as a literal demon, with reference to some of his more infamously ruthless actions.
- Yu-Gi-Oh! has Shien, an Expy of Nobunaga (great warlord, morally unscrupulous, unified his country, died at the hands of his retainer). Shien is also depicted as an ageless Dark-type who wears black-and-crimson winged armor, has glowing red eyes, and eventually became an evil spirit possessing his armor.
- In Vampire: The Masquerade, Oda Nobunaga is said to have been a vampire, who was eventually defeated by werewolves.
- In MapleStory: Mark of Honor, Nobunaga's Evil Plan is to become the reincarnation of the Sixth Heaven Demon King.
- Double Subverted in Onimusha: Warlords: Nobunaga starts out as human, gets an arrow through the throat, and is revived by demons to work for them—but winds up taking control of the demons who wanted him for a lackey! Incidentally, he pulls a One-Winged Angel twice in the second game, once in the third.
- In Persona 2 there are monsters/Persona based on Nobunaga called "Demon King", in Japanese, it's called "Dairoku Tenmaoh". Later, in Persona 4, Kanji Tatsumi's evolved Persona is a shortened version of these demon/Persona: "Rokuten Maoh".
- In Sengoku Basara Nobunaga is, once again, a cruel, ruthless S.O.B wielding a sword and shotgun combo and a nasty cape that can also attack enemies. It's also quite possibly the most Obviously Evil depiction of him ever: a Tin Tyrant decked out in Spikes of Villainy and often seen reclining on his throne of skulls. And when you have Norio Wakamoto as the voice actor, ham is, of course, to be expected.
- The anime turns this Up to Eleven: Dramatic Thunder and Ominous German Chanting accompany Nobunaga whenever he makes an appearance.
- And the third game (and The Movie of the anime) does even better when Nobunaga returns from Hell, having apparently taken over the sixth underworld and turned his epithet into a literal description. As a playable character, his story pretty much involves killing everyone else in Japan before returning to hell.
- Sakura Wars: So Long, My Love has Nobunaga as the Big Bad commanding The Legions of Hell.
- A Japanese-exclusive game for the PC-98, Zan: Yaksa Enbukyoku (A crossover between Yaksa and Zan, two of the earlier Wolf Team's games for Japanese computers) has an interesting twist on this trope: Not only the main characters have to fight against a demonic Nobunaga, they also have to fight against infernal versions of both Tokugawa Ieyasu and Toyotomi Hideyoshi as well. In this case this is semi-justified because the heroes are Christians, but it might not make any sense because Nobunaga is actually a fan of foreign stuffs like Christianity. On the other hand, the MegaDrive version of the same game removes Ieyasu and Hideyoshi from the evil trio, leaving only Nobunaga leading an army of demons in their place, not to mention it also removes any reference about Christianity in the game.
- Downplayed with Nobunaga in Samurai Warriors. He plays the Evil Overlord angle to the hilt and wields a blade shrouded in darkness, but he's big on Pragmatic Villainy, and is really no more exaggerated than any of the other playable characters in the series. The only time he's closest to this trope is in the first installment where he's decidedly more brutal and embracing his 'Demon King' persona, and the installment where his struggles against the Ikko-Ikki sect got a big highlight. Koei rarely touched on that afterwards. Voiced by Juurouta Kosugi here. Outright subverted in the Warriors Orochi series, where he's consistently on the side of the heroes fighting against Orochi. In the 4th game, on the other hand, he likes to live up to his Demon King epithet by suddenly subjugating areas by force in the new world and being antagonistic to the heroes because he's just testing them for the eventual battle against the Greek and Norse Gods, since he was chosen by the Eastern Mystics to wield the bracelet imbued with the power of Japan's creation God Izanagi.
- Subverted in Pokémon Conquest, of all places. Despite being the Big Bad, Nobunaga is actually a rather sympathetic character. Noticeably, his choice of partner Pokémon is Zekrom, which may look like it would put him in this territory, but in the main series lore it is considered not inherently evil, but rather it and its white counterpart Reshiram are both known for having been used in the past by Well Intentioned Extremists and Knight Templars — which historians consider the actual Oda Nobunaga to have been.
- Zig-zagged in Nioh: In this game, he really did mass-murder a great many men, women and children at Mt. Hiei, but treats his crimes as another facet of a rather complicated man, one who was driven by ambition and who also appreciated life and death to the fullest. He demonstrates this by flipping the bird to the necromancer who brought him back, then giving the protagonist his pet phoenix before leaving to the afterlife, even though he was given the demonic power to rule the world uncontested. Most of Nobunaga's former retainers say that he was a strange, unpredictable man. He's voiced by Kou Shibusawa, a major head producer at Koei Tecmo. Notable for the fact that despite following the trope very closely, even as a fabled Demon King, Nobunaga was portrayed not as a megalomaniacal super villain, but more of a Noble Demon.
- In Ninja Masters's Haoh Ninpo Cho, Nobunaga was supposedly dead and then suddenly reappeared as a demonic lord, having made a Deal with the Devil with a powerful demon named Haoh. So naturally, the cast decides to go to Nobunaga's Golden Palace to destroy him... In Nobunaga's own ending, however, he ends up conquering the world.
- Ikemen Sengoku depicts Nobunaga as a deeply flawed but ultimately sympathetic person who has quite understandably earned the hatred of multiple characters for his ruthless killing of countless people but also has just as understandably earned the loyalty and admiration of multiple other characters with his charisma, Tough Love for his men, and genuinely well-intentioned ambition to create a world in which no one has to suffer from poverty or class-based discrimination. He has no supernatural powers in this game, but is given the "Devil King of the Sixth Heaven" moniker by Kennyo, a monk who witnessed his killing of other monks and has vowed to kill him in revenge but is portrayed as being Not So Different from him.
- Fate/Grand Order: The Gender Flip of Oda Nobunaga, having originated from a gag manga, has this trope generally Played for Laughs. Nobu tries to be a Card-Carrying Villain, proudly calling herself Demon King, but she's such a goofball (as well as extremely short) that she just ends up looking the opposite of threatening. However, because Historical Villain Upgrade is an in-universe factor that can posthumously warp Servants, Nobu gets a "Demon King" trait which allows her to shapeshift into a more threatening form to fit her reputation. She only uses it to make her boobs bigger.
- The fourth iteration of her event seems to play this directly straight, introducing a new version of Nobunaga, with an Avenger class, at the height of her legend as the Demon King of the Sixth Heaven, doing away with the gag and joking nature she had before. But you have to level her up first, because she starts out as just her wacky Archer form in a new class, followed by somewhat-male Nobunaga, and THEN you get the Demon King... who's still female. And then played with in that her alignment is only Chaotic Neutral to every other Avenger's Chaotic Evil, making her effectively a Token Good Teammate.
- Puzzle & Dragons features two monsters that play off multiple aspects of the Nobunaga history and mythos. Dark Samurai Dragon, Nobunaga and its evolution, Unwavering Demon Dragon, Nobunaga are both part of the Samurai Dragon monster series, both are Attacker and Devil types, and both have Dark and Fire as their attacks.
- Samurai Shodown's take on Oda, Gaoh, actually stays human in his canon appearance as the final boss of V and V Special. However for the non-canon VI, he becomes Demon Gaoh and commits to the trope.
- As opposed to her more gag-natured traits in Fate/Grand Order or Koha-Ace, Nobunaga plays the trope straighter in Fate/type Redline, since it's basically the previously hilarious Imperial Holy Grail War re-imagined in a dead serious manner. She pretty much trades her wacky antics for terrifying battle lust with Slasher Smile aplenty, living up to the term 'Demon King'. And that's without becoming her final Avenger version.