The world of Digimon: The Great Demon War, unlike that of other series is relatively isolated. Its denizens have not had contact with the humans that have accidentally created them and they are watched over by their 'god', Yggdrassil. Yggdrassil, knowing that its charge were still young and immature in their existence chose to hide their world away to give it time to grow before allowing it to begin interacting with the worlds outside of its domain and the Digital World remained a paradise.
But like all Paradises, it was not to last. Corruption began to spread and Yggdrassil charges its angels and its knights to fight this growing corruption to protect the Digital World. In the process, some of these knights and angels were corrupted by the very thing they fought. Lucemon was one such being, and when his corruption was unmasked, he narrowly escaped with his life and disappeared into the shadows. The corruption took him, and he became Lucemon Chaos Mode, the first of the self proclaimed demon lords. Other demons lords arose as well; Lilithmon, Beelzebumon, Daemon, Leviamon, Belphemon, and Barbamon, and under his banner they gathered an army and raged war upon Yggdrassil and his legions.
The war did not go well. Pushed to the brink by the demonic forces, Yggdrassil chose to use his last resort. A weapon known as the X-Program, which would wipe out 98% of the Digimon population, save for those loyal to him. There were those that survived however. Some, such as the Demon Lords themselves, were strong enough to fight off the X-Program's attempt to annihilate them. Others however turned out to be completely immune, possessing what is known as an X-Antibody. Still, without a means to escape the old Digital World as it collapsed around their ears, both Demon Lord and X-Digimon would have been doomed to die.
Then Unification happened.
Escaping through newly linked portals, the remnants of the old world escaped into the Multiverse, where they continue today. Unbeknownst to them however, Yggdrassil's chosen few and their god still exist, having created a new Digital World that they have kept secret from all.
There are roughly three groups of Digimon in these themes, as follows.
The Exiles: Composed mostly of X-Antibody Digimon, with the rare smattering of regular Digimon strong or lucky enough to survive, the Exiles are what remain of the old Digital World. In a strange new world, they have banded together and believe their old world is gone. If you wish to app for a human/digimon partner for this theme, you would be part of the exiles. All factions are open to characters from this group.
The Chosen: The Chosen are those selected by Yggdrassil to inherit the new world and are loyal to their god. They value their peaceful existence and isolation, and are protected by the Royal Knights. For the most part, any characters belonging to this group will be focused on keeping their world safe and isolated from the rest of the Multiverse and will either be Union or unaligned, depending on how well they can trust the Union to keep their world's existence a secret. The Chosen are often at odds with the Exiles, due to their distrust over this strange 'X-Antibody' that granted them immunity to Yggdrassil's X-Program.
The Demon Lords: It is unknown how many of the Demon Lords or their forces survived the utter destruction of the old world, but survive they did. With much of their power base destroyed however, the alliance between them has broken apart and each Demon Lord has gone their own separate way to carve out their own little empire in the Multiverse. As such, they can either be Unaffiliated or Confederate as the apper desires.