World of Darkness

The World of Darkness (闇の世界 Yami no Sekai), also known as the Dark Ocean, is seen primarily in Digimon Adventure 02. However, we get a glimpse of what might be a part of it near the end of Digimon Adventure, when Yamato and Sora fall into Dark Caves.

Digimon Adventure 02
At some point between Digimon Adventure and Digimon Adventure 02, Ken was transported into this world. When he submerged his Digivice into the Dark Ocean, it changed into a black D-3 and he began his change into the Digimon Kaiser.

In ZT13, Hikari was transported there. She encountered what first appeared to be Hangyomon lacking diving gear, with Evil Spirals on their person. However, the Evil Spirals did not seem to be controlling them, but instead sapped their energy. They said the god they worshiped had done this to them. Hikari tried removing them, but an Airdramon with an Evil Ring on it attacked the cave. Takeru, Patamon and Tailmon then broke through to this world and helped Hikari destroy a Dark Tower hidden inside a lighthouse and defeat the Airdramon.

What had been the Hangyomon then mutated into strange creatures, who grabbed Hikari saying she should breed with them (a reference to the Deep Ones of H. P. Lovecraft). Angewomon shot a warning at them and they retreated, saying they would return to their god. In the end of the episode, the sillouetted form of Dagomon rose from the ocean; he is the one those strange creatures worship as god.

Later on, Ken used his D-3's connection to the World of Darkness to open a gate to it, through which he and the other Chosen Children banished Demon.

Connection to the Cthulhu Mythos
Many of the things connected to the World of Darkness are derived from the works of H. P. Lovecraft and the Cthulhu Mythos.

Dagomon is named after Father Dagon, a minor Great Old One named for a Semitic god, though his appearance is based on that of Cthulhu. The Hangyomon-like creatures who worship him have many similarities to the Deep Ones that worshiped Father Dagon, to the point where many fans refer to them as "Digital Deep Ones".

Another odd connections shows up in Demon, who was banished to the World of Darkness. The cloak Demon wears is a Symbol of the Elder golds, considered the enemies of the Great Old Ones by some fans of the Cthulhu Mythos. It is unknown whether this link was an intentional one, though one may gather from this connection that this means Dagomon cannot hurt Demon.