Well, I don't make a distinction between realms of magic, as far as spells are concerned (i.e. a spell is not part of any realm in particular), so my answer may not be what you wanted, but... To me, realms of magic merely define the magical power source and, as such, are arcanists magic-users drawing power directly from the Fabric of Reality, people whose magical energy is the energy tying together Reality itself. Needless to say, you can count them on the fingers of one hand, even across the hundreds millennia since sentient beings had existed in my world --including when taking into account the gods! ^^;;;;;