The Verum Transitus resides within a castle on the world of Erebusa within Void Space. The castle itself houses an extra-dimensional pocket realm which contains endless passageways and tunnels, both above and below ground. The castle itself is ancient and timeless, the interior of the castle is empty and without any living entity at all. It is also known as The Crossroads. Upon entering the castle from the outside, one finds themselves in a central great hall, from which all of its staircases and hallways originate.
Within the castle hallways branch and turn infinitely, each one leading eventually to a different point in the multiverse. One has to know how to navigate the castle in order to get to the point in time, space, and reality they wish to go. Each branch of the hallway is marked by a different series of arcane runes, these runes form a guide though their original language has been lost to time. The runes allow a person to navigate the endless hallways, staircases, and pathways.
When one knows where they wish to go, the path becomes clearer and the hallways shift and move before you to make your path easier and quicker. This effect works in reverse, upon arriving within the crossroads, one quickly finds the path away from the chamber leads back for the great hall. From the great hall a person is able to follow various paths to other places in the multiverse.
The only known map, of any kind, to the Transitus can be found in the pages of any of the three authentic Rose Codex books. Maps can be made based on the routs within its pages and runes recorded to lead from one place to another, but only the codex itself can find any door to any place at any time.
The castle does not contain portals exactly, but rather hallways end in circular rooms with a crystal at the center of the room, the walls with bands of arcane runes. Once inside the room, the crystal is activated by touch where it then teleports the person through the networks of connected realities to their destination. Transit to the Verum Transitus is accomplished much the same way, there are sites throughout the worlds in the realities which act as natural conduits, but one has to know the secrets to activating those sites.
The secret to activating one of these sites lies in knowing the reality you are in, the arcane runes within the Verum Transitus which guide a person to each reality form the runes which are used to call upon the site to teleport you into the Verum Transitus itself. The runes each have a pronunciation, though the language has long since been lost, they can be spoke in this way as arcane words or drawn upon parchment, which acts as a portal key. Once used such a key vanishes.
Rooms exists in much the same type as they do in the castle. No one is quite sure how but points appear in buildings of any age, tunnels, even caves as needed. It is completely unpredictable where one will be, but points of entry will form of their own accord in locations as time goes on. Some locations register as thousands of eons old while others seem to be newly made.
A research study of locations on Earth done in 1997 referenced location appearances in the modern world. They found that blueprints shows building these rooms in various office buildings where such places existed at different times. For each case, there are no notes of where the idea or design of the room came from, nor does anyone mention it appearing unexpectedly. Notes from construction sites show that the rooms were on the blueprints and this was normal somehow. The earliest records go back to the early 12th century and whenever one such site is destroyed, another appears in its place.
The Under Realms
Paths within the crossroads can also lead to places not within the material plane, these are accessed via the underground passageways, they work much the same way but are darker and far more foreboding. These pathways are far less friendly to visitors and one must know exactly where they are going or they risk being lost forever.