How They Work Edit
Portals do not open directly to their destination - instead there are actually two portals that compose a Corridor of Darkness - an entry portal and an exit portal.
The entry portal is opened nearby the traveler. This portal actually opens up into either the Realm of Darkness or the Realm of Nothingness, depending on who is using it (Nobodies open into the Realm of Nothingness, all others open into the Realm of Darkness). A second entry portal is opened by the user where he or she wants to go - so there are now two portals opening into the Realm of Darkness/Nothingness, from different locations. This second entry portal is used by the traveler to exit out of the Realm of Darkness/Nothingness to the new location, thus becoming an exit portal, and allowing it to be used both ways.
For example, say Raxen wants to take a trip to Atlantica. To do so, he opens a portal into the Realm of Nothingness next to him, and simultaneously opens a portal in Atlantica. He steps into one portal, passing briefly through the Realm of Nothingness, and into Atlantica.
Expert Use Edit
With practice, a Corridor user can virtually eliminate passing through the separate Realms. By opening the entry and exit portals as close as possible within the Realm, the traveler can step through both portals at almost the same time and simply come to their destination. However, this takes practice and concentration to do properly. Many early users may need to open their second portal after they've entered the new Realm in order for it to work properly.
Though some Corridors pass through the Realm of Nothing rather than the Realm of Darkness, all portals are composed of pure Darkness. As such, all portals possess some threat to their users when used too often. Simply passing through the portals can leave some hint of Darkness behind on and in a person, which can slowly wear away at their Heart. If used too often without protection, that person's Heart can become weak enough from the Darkness that any contact with Darkness will cause them to become a Heartless.
Further danger comes from passing through the Realms. The Realm of Darkness is even worse to pass through than the portals themselves, and eats away at a Heart's Light even faster. The Realm of Nothingness is not without its dangers as well, as it eats away at a person's very existence. Prolonged time in the Realm of Nothingness can cause a person to begin to fade. These empty cavities of existence can be filled with darkness when using an exit portal, slowly gathering Darkness in the body.
Nobodies are not exempt from danger, even though they lack Hearts. Their bodies possess large amounts of Nothingness which can be filled by the Darkness of the portals themselves, or by the Realm of Darkness. As well, they are very susceptible to the diminishing effects of the Realm of Nothingness.
Continued use of portals unprotected can cause the loss of an unborn child's Heart, or cause it to never develop in the first place.
However, most of these dangers can be avoided simply by wearing a Black Coat when traveling through the Corridors of Darkness, as they prevent Darkness from entering or affecting the body.