World of Darkness

The World of Darkness (暗黒の世界 Ankoku no Sekai, 闇の世界 Yami no Sekai) 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 Dagomon's Ocean, it changed into a black D-3 and he began his change into the Digimon Kaiser.

In Digimon Adventure 02 - Episode 13, 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 silhouetted 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
See Cthulhu Mythos references in Digimon