Ryuuma-ou-Mikado (the Evil Dragon King of Hell) is a major villain and very likely the main villain of the Kishin Douji Zenki series. He is a gigantic black dragon with several heads.
Ryuuma-ou-Mikado is exclusive to the manga. Little is known about him, but he was and is a major threat to our world, which Zenki fought in a fierce battle about 1000 years ago.
They took over and Ozunu managed to seal Ryuuma-ou-Mikado, ending a great war between humanity and the monsters and returning Japan to a peaceful state.
More about the battle against Ryuuma-ou-Mikado can be read in our entry about the ancient Zenki.
Manga (Volume 12)
While Ryuuma-ou-Mikado starts out in his sealed state, he is soon visited by a small group of beastmen. They kidnapped Saki in an attempt to force her to help them unseal Ryuuma-ou-Mikado.
Luckily this plan doesn't work out for them as Zenki, Chiaki and Goki arrive on the scene to stop them, but the beastmen still have an ace up their sleeves as their leader Kokuu, a mysterious young swordsman, has been casting a complex spell to unseal the dragon as well.
After they saved Saki, our heroes return to the city, while unbeknownst to them, Kokuu keeps casting his spell. While they are riding the bus, Akira tells the others about the battle between Ozunu and his guardians and Ryuuma-ou-Mikado.
We then see a flashback of Zenki having lost against the dragon and Ozunu, Goki and Vasara joining him and taking over. Ozunu then succeeds in sealing Ryuuma-ou-Mikado.
The scene cuts to a large crowd of beastmen and beastwomen cheer on Kokuu. They hope that Ryuuma-ou-Mikado will lead them to a new age where they can eliminate humanity so they can rule earth in our stead. This fails when Kokuu succeeds to unseal the dragon, but Ryuuma-ou-Mikado attacks the beastmen and beastwomen instead of helping them.
When Zenki and Goki hear their screams, the heroes immediately head to the city where Ryuuma-ou-Mikado can be seen on a rampage, tearing down large buildings and attacking the people.
While Zenki, Goki and Chiaki go ahead and prepare themselves to Ryuuma-ou-Mikado, a group of various monks and Shugenja lead by Miki Souma and Saki are preparing a pentagram seal to seal the dragon once again.
Zenki, Goki and Chiaki leap straight into a fierce battle against the dragon. When Ryuuma-ou-Mikado tries to chomp Chiaki, a vision of Ozunu appears and sends her a new battle armor to show her that he has accepted her as his successor.
The battle continues, but the heroes are losing. When Chiaki gets attacked again, the winged, feral boy Dokushi jumps in and helps her. He cries manly tears and tells her about how Ryuuma-ou-Mikado killed his friends.
While they now continue fighting the dragon once again, the scene cuts to Miki Souma, who calls upon Vasara. When this isn't enough to free Vasara, Kokuu appears and cuts open the rock which sealed Vasara, freeing him and allowing him to participate in battle.
When Ryuuma-ou-Mikado targets Chiaki once again, she is saved by Vasara. At first, Chiaki, Goki and Zenki are shocked and believe that Vasara is going to give them more trouble, but when Zenki shows him his fierce determination, Vasara joins them.
As this seemingly still isn't enough to defeat the dragon, Kokuu shows up once again and hands Chiaki his sword. But then it turns out that the sword is a part of his body and he dies.
Dokushi, who had previously seen Kokuu as a brotherly figure, cries out in agony over the loss of his friend and the scene now shows Vasara, Goki, Chiaki, Zenki and Dokushi facing Ryuuma-ou-Mikado together.
They attack the dragon and kill it in a huge explosion. The threat is taken down and everyone smiles as peace returns to the city.
The chapter and the volume end with another shot showing Vasara, Goki, Chiaki, Zenki and Dokushi.
While Ryuuma-ou-Mikado has previously been thought to be a manga-exclusive character, he actually makes a cameo in Kishin Douji Zenki - Battle Raiden for the SNES.
Ingame, the dragon is known as 竜脈を (Ryūmyakuo), though this is only present in the original, Japanese version of the game, where Shijura's ingame counterpart states the following:
"This is Ryuumyakuo, sealed by Enno Ozunu inside what is now known as the Dragon Emperor Hill." (translation by Zenki)
Names and minor mistakes
While Ryuumyakuo directly translates as "dragonfly", its individual Kanji and Furigana can also be translated as "the Dragon Pulse" or "the Dragon's Heartbeat", which is likely the reason, why his name was omitted in the English translation by DynamicDesigns.
An inversion of this can be seen with Anjura's ingame counterpart, who doesn't have a name in the original game and becomes the "Dragon Princess", as her line has been accidentally "translated" into a name.
Ryuumyakuo's existence has been overseen for a very long time, as one has to not only compare one specific page from the manga, namely the one showing Dokushi, with the ingame screenshots, but needs to know that Ryuuma-ou-Mikado exists, so they can draw the line between him and his ingame counterpart.
This line becomes even clearer, as Ryuuma-ou-Mikado can be translated as "Evil Dragon King of Hell" and his ingame counterpart, Ryuumyakuo, is sealed inside the "Dragon Emperor Hill".
Not only do both dragons bear similar names, the "Evil Dragon King" and the "Dragon Emperor" also aren't too farfetched from each other.
In Battle Raiden's intro, Shijura, a villain from the earlier volumes of the manga, and his followers find the Dragon Emperor Hill. Shijura seeks to free the dragon from Ozunu Enno's seal, so he can control Ryuumyakuo, so he can use the dragon's power to become the supreme ruler of our world.
When Shijura tries to break the seal, he fails, but then says, that he didn't expect anything less from Ozunu Enno's seal. Shijura then states, that if they won't let him break the seal so easily, he is going to take his time.
Not only does he mention Ozunu in relation to the Dragon Emperor Hill, he also tries to gain control of the dragon, so he can use his power to become the supreme ruler of this world, which is exactly what the beastmen and beastwomen in the manga are trying as well.
Now, while the motives of the Jushi and the beastmen are different, - the Jushi want to prove that they are superior sorcerers to Ozunu's bloodline, while the beastmen want to overthrow humanity - they both try to grasp Ryuuma-ou-Mikado's power by releasing his seal.
After beating the game, Chiaki tells Zenki how Ozunu's dream prediction won't come true, but Zenki tells her that he wouldn't be so sure about that, stating that "As long as humans continue to have desires, the same thing will happen again and again!", which makes a continuity nod towards what happens in the manga.
Only, that it can be extended to the beastmen as well, as they don't just have a human- or Kishin-like thought process, but also do exactly the same thing, that Shijura tried way earlier.
Strangely, Battle Raiden explores details of the Jushi ark, which aren't seen in the manga, while the manga has more in-depth content on the more obvious side of the story.