The Dark Places
Oubliette. Umbra. Shadowlands. Dreamscape.
That character was one I never got around to introducing in a game, but she was always intended to be one of those mysterious quest giving figures, unable to manipulate the real world and needing agents to fulfil her agenda. There might be others working against her too.
The spheres work as shells of stability, an innermost shell is the mundane world. A second shell is populated by the people and things you see out of the corner of your eye, daydreams, things that can be pulled into reality with a bit of effort but aren't quite there yet. A third shell is less tangible still; echoes of echoes, realms of myth and legend that remain in orbit around the collective subconscious, cryptozoological creatures, and things unprovable by science. A fourth shell is stranger still.
In Cyberpunk 2020, a hacker could traverse an abstract digital cyberspace, or could enter a virtual reality. This concept is similar, with an astral traveller or spirit traversing a dark ectoplasmic realm shaped by consensual belief and moulded from the stuff of souls, but at certain points there is enough regular belief or dreaming consensus that a stable pocket realm forms... maybe a goblin market, a mountain fortress of the gods, an eldritch city of non-eucledean geometry, a lost city of gold. It's all there if you know how to look for it.
Familiars would be denizens from those stable realms, somehow caught in the mundane, but holding onto a fragment of the arcane and serving as a catalyst for those with the potential to shape reality (whether mutants, mystics, or agents of the Law).