Magic functions differently than in core D&D. In core, mages freely conjure spells with no actual link to the realms they correspond to. For example, casting fireball does not interact with the Plane of Fire in any way. Not so here.

Mages most commonly draw their power by tapping into the power of a Warren. A Warren is an entire plane of existence. They have geography and sentient and non-sentient life. The majority of Warrens have a ruling being that reigns over it, most often a god. There exists a racial Warren of each race, exhibiting their own unique effects and properties.

Think of a Warren as a closed door in the mage's mind. It's always there. Accessing the power within requires a deliberate and careful opening of the door. Controlling an open Warren is a tiresome and draining procedure, and if opened too far it can cause physical harm, madness or even death. Racial Warrens have long since been lost to history and no known mages exist which can access them.