To access the Dark Dimension from Earth, there must be access to gates called the Kimon Gate or more often, the Demon Gate. One of these is the Demon Gate of Sedona, which is located in America. It is located in a warehouse, that leads into a cave outside of the City of Darkness. On Earth, there are thousands of gates, which are located in the intersection between energy lines. Most of them in America were closed or have fallen into disuse after the arrival of Europeans and the disappearance of the Native Americans. Although some still remain in Europe and Asia, one of the most prominent is in Japan, run by the Kitsune, like the Gate of Sedona. The Demon Gate leads to the City of Darkness.
The rulers of the Dark Dimension are the Guardians from the Celestial Court, which impose the order. The summit of power, however, is the Superior Court, which decides everything and has the power to restore, revive or kill. Some believe this court has become corrupt, and that the Guardians are no longer fair in their rulings...
Read more on each realm in their respective descriptions.