Tartarus

Tartarus are the rifts which appear in Monster Girl Quest: Paradox. In the Paradox Trilogy universe, thirty years before the game's events, a catastrophe which is known as the Great Disaster drastically altered the entire world in the same day. As a result, seven huge black holes called Tartarus appeared all over the world; each Tartarus had massive dungeons within, filled with Apoptosis, and lots of highly advanced relics and technologies called Makina, or Machines, were discovered.

As evidenced by the destruction of Heaven, the Tartarus literally tore through the fabric of reality itself. It's not just like a massive explosion created them. A piece of something from another world tore through into this one and built the pits which the inhabitants of the Paradox world refer to as Tartarus.

Each of them include a mysterious door that can be only opened by a small number of people that can travel between dimensions, including the hero Luka. Even Ilias note he shouldn't be able to do so.

Behind these doors there is a part of the destroyed city of Remina (except for the one located in Sabasa), and a place known as the Chaos Passage with a portal somehow leading to another dimension...

Chaos Passage
These mysterious places can be found behind the doors present through the Tartarus Rifts around the world. They look like paths of dirt floating in the middle of the void, each leading to a small altar with a portal leading to another world. No one knows the reason to be of these places, and there is not a single sign of life to be seen.

Known Tartarus lists

 * Ilias Cont. South Tartarus
 * Ilias Cont. East Tartarus
 * Sabasa Cont. North Tartarus
 * Grand Noah West
 * Gold Cont. West Tartarus
 * Central Centora
 * Former site of Remina

Trivia

 * Tartarus, the Greek underworld, is based off Greek mythology. Tartarus was two things. It was the Greek underworld, and also a primordial force, the ancestor of the Titans. While Hades was the place of the dead, Tartarus was the place where the worst of the worst went, such as the Titans. Think of it like Hell, the opposite of Heaven. Tartarus being deeper than Hades itself would connect to the fact Tartarus is the deepest location accessible in the Paradox world. There were five other primordials, but whether they will be mentioned in Paradox cannot be said.
 * Luka feels a sense of familiarity the first time he enters a passage. It is unclear if it is related to Hades or foreshadowing.