"What the heck is this Arcanus Obliques thing?"
Thank you! I'm glad you asked.
Short Answer: Your character doesn't get to go there. Sorry.
Long Answer:
The Arcanus Obliques is simply a collective term for all of the various other planes of existence within our own native 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 Planes are obviously pretty rare in storytelling, but they help form the basis for many of the supernatural elements of our stories. Having said that, you may see these mentioned, but will rarely if ever get more information than that. Consider this post the one and only exception. If you get interested in one and want to explore it in your own writing, be prepared for some pretty cutthroat revisions.
The Arcanus Obliques, and the physical universe as a whole, are part of a system that can best be likened to a picture of aligning waves with the same wavelength but different amplitudes; that is they all meet at three distinct points: The Inner Horizon, The In-Between, and The Outer Horizon. Both horizons border infinity (in the truest sense) and serve as boundary markers for all known reality, on any level. The entirety of our universe exists between them. The In-Between, on the other hand, exists in the exact middle of everything. Because of the rather elastic nature of the middle the In-Between is the most well traveled fixed point in all causality. The precise midpoint of every journey, for example, passes through the In-Between. So does the middle of every lifetime, and the middle of every chocolate bar. Try not to think of it in terms of physical space, or you'll get supremely confused.
Let's shift gears for a minute, and discuss some of the various planes of the Arcanus Obliques. Here is a brief description for some of the more popular planes:
The Astral Plane: The wellspring of most of the non-elemental magic on earth. Any visit to the Astral Plane is a dizzying foray into folded L-Space. This place exists with rather more dimensions than the traditional three. (It is quite a popular place to keep hammers and mallets.) Many of the Indefinites (vast powers without known form or purpose) inhabit the Astral Plane, and it is theorized that some of the world's most powerful magical entities (Gods, D'jinn, etc) started their existence as Indefinites.
The Elemental Planes: There are dozens of these, and they all suck. The Elemental Plane of Fire, for instance is just that. Fire. Just a big firey realm with no air or matter or anything but fire fire everywhere for infinity in every direction. Likewise the elemental plane of water isn't at all like an ocean, because even oceans have limits, (and fish.) Easier to see than the Astral Plane, but far more difficult to control. You don't visit these places unless you're an elemental yourself, or some kind of pan-dimensional hyper being, in which case you'd probably get bored with them very quickly since there really is nothing to see. For everyone else, these don't even represent a useful way to access said elements, since opening a small portal to something as inconspicuous as the Elemental Plane of Air could potentially vaporize the planet. They exist, but try not to think about them.
The Ethereal Plane: The razor thin (and lightyears wide) bridge between life and death. The Ethereal Plane is inhabited by all kinds of Things from The Other Side. Ghosts and shades do a lot of mucking about here, mingling with creatures of shadow and thought. The Ethereal Plane is sometimes wrongfully characterized as the Nightmare Plane.
The Psychic Plane: This place is a wreck. Various secret wars fought here have left the once-pleasant thoughtspace looking like a Salvador Dali painting. Still, it can be visited, and traversed, if you don't mind (heh) the psionic equivalent of crawling over broken glass.
The Temporal Plane: Home to nobody, the Temporal Plane represents a sort of map of time. It's probably best to mention now that this place is quite impossible to get to, as it exists outside causality. A casual misstep here can easily result in not having ever existed in the first place. Time manipulation is one of those off-limits powers in the FPL, but speedsters approach lightspeed can get a glimpse of the Temporal Plane (in that picosecond just before their bodies become made of energy and dispersed evenly throughout the cosmos.) Why mention it if nobody can get there? Because its existence is quite necessary!
Heaven and Hell: Yeah, they exist, branching off in as many bifurcated variations as you can imagine, and several you thankfully can't. You've got to die (or be of a suitably angelic or demonic stock) to visit them, so don't even try.
Whew. To make things more complicated, each of these planes has a profound effect on people sensitive to it, and bears a distinct relationship with the others (except the Elemental Planes, which totally bite anyway.) For example: specific points in The Psychic Plane allow glimpses of the Temporal Plane, possibly letting one derive the future. Similarly Ethereal forms may bleed over onto the Astral Plane, allowing the magically sensitive to commune with the dead. One doesn't need to physically visit these planes to achieve said effects, but an understanding of the basic fabric of this reality doesn't hurt.
So there you have it, an overly long and confusing explanation for a whole set of rules you're not even allowed to use under most normal circumstances. Isn't life grand? By the way it should be explained that it is entirely possibly for, say, a powerful psychic character to go his or her entire life without once visiting the psychic plane. Ditto for the respective planar relationships of mages, ghosts, demons, and whatever being made entirely out of Higgs bosons makes you. The physical realm is a nice and comfortable place. I'm quite fond of it myself.
