As it turned out, my last theory was right. Somewhat, at least.
Hellspace was not a portal to the Nether. Instead, it connected me to another space. A space that had been created for none other than myself. And for my army of demons, of course.
Trickster was the first to barge through the fracture in space. His soldiers followed slowly, filling a portion of the Hellspace with ease. Closing the fracture once the entire army was in the Hellspace, I walked around a little, curious if someone would sense something.
I left the Eserian as a Gale Wolf and returned to Cintre where I shifted back to my main form. Stepping back into the fortress, I half expected to be stopped and to be eyed weirdly. That didn't happen.
