World of Darkness

The World of Darkness, also known as the Dark Ocean, is seen primarily in Digimon 02. Though we do get a glimps of what might be a part of it near the end of Digimon Adventure when Yamato and Sora fall into Dark Caves.

At some point between Adventure and 02 Ken is transported into this world. When he submerges his Digivice into the Dark Ocean it changes into a black D-3 and he begins to change into the Digimon Kaiser.

We first see it in episode 13 of Digimon 02 when Hikari is transported there. She encounters what first appeared to be Hangyomon with Evil Spirals on them, but the Evil Spirals did not seem to be controling them but sapping their energies. They said the God they worshipped had done this to them.

She started to try and remove them but an Airdramon with an Evil Ring on it, or at least it appeared to be one, attacked the cave. They ran out. She called out to Taichi, Tailmon and Takeru. Takeru, Patamon and Tailmon then broke through to this world and helped out. Under Tailmon's instructions Takeru armor evolved Patamon to Pegasmon and destroyed the Dark Tower hidden inside a light house that shown a dark beam. This enabled Tailmon to super evolve into Angewomon and easily beat the Airdramon.

What had been the Hangyomon then mutated into strange creatures (not made clear wether they are Digimon or not) and grabbed Hikari saying she could be their wife. Angewomon shoot a warning shot at them and they retreated saying they would return to their God.

There god is rumoured to be Dagomon who is based on two of HP lovecraft's old ones. These are Dagon and Cthulhu. Dagons servants are the deep ones which are what the fake digimon are rumoured to be (digital deep ones).

Later on, Demon is banished here. Another odd connection is that on Demon's clock is a symbol of the elder gods (the enemys of the old ones) whether this was a mistake, or an intentional link we don't know. Though one may gather from this connection that this means Dagomon can't hurt him.