The Realm of the Dragon King is a continent found in Final Fantasy Brave Exvius and the final one to be featured on the world of Lapis. Canonically the Realm of the Dragon King appears after the end of season one and serves as an interlude between that point and season two. The strongest of all Espers and member of Hess' Eight Sages, Bahamut makes his lair on this location.
Following the battle with Sol and the Chaotic Darkness, Rain disappears on his clash with Sol. As the party returns to Zoldaad, they talk with Evan who speaks about a mysterious floating island causing distortions and wild storms which make travel for most airships impossible. He offers to take them in the Invincible which can ride up to there.
Upon reaching the new mysterious land, a Vision of Rain manifested unconsciously by the party guides them through the continent where Fina is able to sense the presence of the fellow sage Bahamut who has taken lair on the island. Sakura mentions he's the strongest of all Espers both in power and intelligence, this motivates Lasswell to seek an audience with the Dragon King in order to seek his help for their quest to find the true Rain. The Vision of Rain continues to guide them through a perilous land until they reach the Eternal Summit.
On Bahamut's throne the Dragon King greets his visitors not being able to remember Fina as he has partial memory loss from being sealed in Crystal. Lasswell trying to test his own ability to have Espers side with him as Rain previously had, asks the Dragon King for his strength. The Dragon King is not surprised as his power has been sought by many. Lasswell and the party then voice their intentions to help them find Rain. Bahamut ridicules their motives as insignificant and recalls that humans of far higher position and power have come to him for reasonably much bigger causes. This prompts Lasswell to draw his weapon in order to show the Sage the conviction within his emotions.
The party following Lasswell engages in combat with the mightiest of Espers and are able to defeat him after a difficult battle. Lasswell and the party share with Bahamut how important Rain is to them and why they request his help in order to find him. Bahamut remembers how no matter the age he was called in, the emotions of his summoners were always the strongest thing he had ever witnessed. Having now seen the conviction within the party, Bahamut deems their lofty cause just and agrees to join them.
In order to unlock the Realm of the Dragon King, the player must beat all the content of season one, namely defeating the Chaotic Darkness. Subsequently they must go to Downtown Zoldaad in Zoldaad and speak with Evan who will talk about a distortion, then upon returning to the world map the Realm of the Dragon King should be visible in the continent selection (located on the lower right part of the map, and being quite small, however).
Travel in the Realm of the Dragon King differs significantly from other continents. For one the player doesn't play stages in order to progress, rather the Realm of the Dragon King is one giant map consisting of several areas all interconnected in some way and creating a path towards Bahamut's lair. As such the Realm of the Dragon King can be considered a massive Exploration.
Therefore, although it is technically possible to travel the entire continent in one go, it is at large considered an unwise practice. Instead what the player is recommended to do is set camp bases. To do so the player must carry Tents in their item inventory and upon reaching certain safe spots in each area (there is one in all of them, except the starting point and Eternal Summit, where Bahamut is found which is short). Upon doing so the player is given the chance to set a camp, which will cause them to exit the exploration and earn their rewards as usual, but with the added benefit that they will be able to start exploration from any of the camp sites, thus serving as effective checkpoints.
As stated above, this is not mandatory to do, but nonetheless an almost needed action to perform. The reason for this is that the map as a whole is incredibly extensive and the random encounters are significantly stronger than any faced at this point, ambushes are frequent and the monsters are specially scripted to be able to attack many more times than usual each, and be capable of even chaining their attacks. As such a bad luck encounter can easily kill even the most prepared of parties without giving the player any chance to even fight back. This is dangerous as unfortunately for the player, Lapis cannot be used to continue, so a Game Over will mean forfeiting all progress achieved.
It is thus in the player's best interests to set camp as frequently as possible as they progress the map. Equipping any ability that reduces Random Encounters is also valuable to make exploration easier (Diabolos, Charm Bangle, player's units passive abilities, etc.) Smoke Bombs or the Escape ability also help. The player should also bring items like Phoenix Down and others in case things go south and mobs kill the healers.
Also even if the full map can be explored in one go, each Area (signaled as icon on the World Map) has its own set of Missions that can only be achieved when entering from that particular spot. So for full rewards, the player is required to enter all of them individually.
Unlocking the Eternal Summit is not enough to face Bahamut, the player also needs to bring with them Rat Tails and put them in their item inventory. One Rat Tail is offered each time the player opens the path to face Bahamut. In order to acquire Rat Tails the player must defeat the Shadow Bahamuts found in the other areas. They always appear in specific spawn points and will always have one Shadow Bahamut per area at a time. Hoarding them as the player makes progress through the map is a good idea as it will save them NRG and time.
Do note however that Rat Tails won't be available for use until the player exists the dungeon and equips them in their inventory. Therefore it is impossible to collect Rat Tails and face Bahamut in one full go.
Bahamut is a terrifying opponent and leagues above in difficulty than the rest of the story content, so even well equipped parties can be brought down quick. His high HP and offensive abilities make it difficult enough to be considered a Trial on his own right.