The Arcanus Obliques is simply a collective term for all of the various other planes of existence within our own narrative reality. Many are deceptively easy to get to (for those who know how) but much harder to return from, just as it is much easier to jump off a cliff than it is to climb back up. Others are entirely inaccessible. Some methods of teleportation actually involve using specific spots in the Arcanus Obliques as an intermediate point. These types of places are obviously pretty common in storytelling, but to help form a common basis for our stories feel free to utilize established planes. If a story requires a non-physical dimension, it is a part of the Arcanus Obliques. It takes something special, be it decades of intense study and focus or a chance dimensional rift, for a regular person to visit the Arcanus Obliques. The inverse of this is just as true.