Core

The Core is a magical, underground realm believed to be the daytime home and source of demons and demon magic. It is not a physical place, per se, as you can't tunnel down into the earth to reach it, but appears to be accessed if sufficiently infused with demon magic and a "path" down can be found. Most demons, naturally, can travel to and from the core, but some mechanism exists to ease their passage from the core at night, but prevents them from returning at will until near dawn.