I was thinking about writing a section for this on the site the other day since I get asked so much about it, but I couldn't really think of where to put it. So I decided it should be an article. Why not!?
Now, if you have never heard of the Dark Ocean, or the Dark World, or the Dark Area, then you must be living under a rock. The Dark Area is a concept in Digimon that has existed almost as long as the show itself. We were introduced to the Dark Ocean (the dub name for the Dark World), or the Dark World in Digimon Adventure 02 episode 13, "Call of the Dagomon", or for the dubbies out there, "His Master's Voice". Subsequent episodes also referenced or took place in the Dark World - 19, 23, 31, 45, and 50 that I recall. In the dub, the Dark World was largely unexplained, but we had a but more information in the original...but still, what is it you ask!?
But before I answer that question, let me tell you about the Dark Area. We saw a version of the Dark Area in Frontier, but let's assume that not all Dark Areas look like that. From other sources like the cards, the games, etc, we know that the Dark Area is an area filled with evil Digimon. When a Digimon dies, Anubimon supposedly decides whether to reincarnate it in the Village of Beginnings, or send it to the Dark Area. The Demon Lords are also reputedly confined to the Dark Area. So, the Dark Area is a place where evil Digimon are sent upon their death, and where the uberpowerful Demon Lords are sealed away. Remember this.
And now, what do we know of the Dark World? Unfortunately, the original version of Call of the Dagomon is not available to me, but the only key difference I know of is that the Hangyomon (Scubamon/Divermon's original name) is that either Dagomon or the Hangyromon didn't want to make her a Queen in the way the dub suggested...they wanted to procreate with her. After this failed, Dagomon appeared for a brief time. More importantly, the Hangyomon turned out to be not Hangyomon at all, but some sort of spirit creature. However, the Dark World must be somehow related to the Digital World; how would they know to look like Hangyomon? Why would Dagomon automatically assume the form of a Dagomon? What about the Airdramon?
A few episodes later in episode 19, "An Old Enemy Returns"/"The Synthetic Demon Beast Chimeramon", the Digimon Kaiser goes into the "Dark Whirlpool" to harvest some of Devimon's data. Note that before the Kaiser even begins, some of Devimon's data is already missing, and also that there are spirit-like creatures everywhere around the whirlpool, which is quite obviously a portal to the Dark World. Chimeramon is then created from some of this incomplete data, which possibly explains why he went out of the Digimon Kaiser's control.
Episode 23, "Genesis of Evil"/"When the DigiVice was dyed in Darkness", we see a part of Ryo and Ken's adventures, and we know that in the end, Ken was hit by a Dark Seed. This is also canon for the anime, as we see proven in episode 44. We also know that when Ken used the Digivice the first time, he was brought to the regular Digital World - but when the Dark Seed was presumably activated by Osamu's death, it brought him to the Dark World, where it transformed his Digivice into a Black D-3. Note that there was a dubbing mistake in this episode; Ken is shown in the Dark World saying it's the first time he was in the Digital World, and then later he says he must remember the first time he went to Digital World, and he remembers Ryo and Wormmon in the Digital World's desert. But why would the Dark Seed cause Ken to arrive in the Dark World? Was Millenniumon, the Digimon who planted the Seed into him, related to it...?
The next important reference is episode 45, "The Dark Gate" (both dub and original title). Ken opens up a portal to the Dark World with his Digivice, and in the original version Demon exclaims "Are wa...Dagomon no umi!", or "That's...Dagomon's Sea!". This strongly implies Demon had been in the Dark World before, and what's even more interesting, he knew of Dagomon. Remember that Demon happens to be a Demon Lord; indeed, one of the Seven Great Demon Lords.
Now, the spirit-like beings in 13 and 19 could be actual spirits. We know Digimon have them - look at Belial Vamdemon, Moon=Millenniumon, Zeed Millenniumon, and Wizarmon. Moon=Millenniumon and Zeed Millenniumon don't just have spirits; they ARE spirits. If the Dark World and the Dark Area are one and the same, this would make sense - when Anubimon, or whoever, chooses the Digimon to be sent to the Dark World, they DON'T reincarnate them or heal them. They're stuck as they are, missing data. In some cases, maybe they're not missing all their body, like Devimon, who was just missing his legs. Maybe the reason the Hangyomon appeared as Hangyomon is because the Spirals FORCED them into having bodies, or maybe they were killed while wearing the Spirals. Anyhow, the whole Digimon could be Digimon that have evolved from the incomplete Digimon. Demon would know Dagomon because he had come from the Dark World - he was a Demon Lord, after all.
See! It all makes sense!But then, if so, you ask, how did Demon escape? Why didn't he return? Just what was Millenniuimon's connections to the Dark World!? I propose that perhaps Demon was a former subordinate of Millenniumon - we do know he had a Demon under his command from the games - and he was able to escape because of the weakened seal on the Dark World that was also affected by the destruction of the Holy Stones. He came because of Vamdemon's interference with the Dark Seeds, and he did not return because the combined forces of Imperial Dramon Fighter Mode and Omegamon, and later Imperial Dramon Paladin Mode, would be enough to defeat him, and plus Vamdemon was dead.. Additionally, the defenses of the rest of the Digital World would have been repaired after a while. Millenniumon himself was the very incarnation of the powers of darkness, and it's possible that the Mugen Dramon and Chimeramon he is composed of jogressed there, and when his body was deleted that could be where he was returned to. And with his powers, it certainly wouldn't be any trouble for him to hop around through sealed-off areas. Maybe the Dark World was the last place where Millenniumon's wishes were being carried out...
I wonder though...Demon has the Elder Sign on him, and Dagon was a priest of Cthulhu, and Dagomon himself looks like Cthulhu. The Elder Ones had the Elder Sign to protect themselves from Cthulhu. Could Demon and Dagomon be at war? Sure, Dagomon is weaker than Demon, but maybe he has a bigger army than Demon - after all, we've seen that Perfects can at least contain the attacks of Finals/Ultimates...