Realm of Titans
Magic and Mages
Magic at its core works much the same way as in a usual D&D campaign, with different planes of existence being used to provide energy for the spells, but the planes themselves are very different. Currently, the only known planes from which energy can be drawn are the Aether Plane and the Abeyant Plane, although there is no reason to believe that these are the only ones that exist. The plane that humanity inhabits is known as the Material Plane.
The Aether Plane is a plane of roiling energy that is the most common source of energy used by mages. The Titans possessed the ability to draw energy from this plane freely, and their deaths weakened the barrier between the Material Plane and the Aether Plane enough that humans and other lifeforms could draw on its energy as well. Some even theorize that this is where the Titans originated.
The Abeyant Plane is a plane of mostly-dormant, yet extremely powerful energy, packed so densely it actually forms a solid landscape. Abeyant energy is far more powerful than Aether energy, yet it is also more unstable, and every time the plane is drawn on for energy, it is brought slightly closer to a violent upheaval. For this reason, the use of Abeyant energy to fuel spells is highly discouraged, if not outlawed, by most of society. While nobody knows for sure exactly what will happen when the Abeyant Plane erupts, there is little doubt that it will be a highly destructive and disastrous event. Those who regularly use the Abeyant Plane for energy are known as Abeyancers.
Unlike a normal D&D campaign, however, the ability to use magic is not something that can be taught. Education and training can further one's abilities, but if you aren't born with the ability to use magic, nothing can change that. Anyone with the potential to use magic is known as a Mage, even if they have no formal training or have never cast a single spell. Every Mage is associated with an element corresponding to a Titan, and that Titan's sigil will appear as a birthmark somewhere on their body. This element doesn't limit the type of spells they can learn, but it can alter some spells' effects.
Mages of a certain element occur most frequently near the corpses of the Titan associated with said element, although there are rare cases of Mages born far away from their element's Titan. There have also never been any recorded instances of a Mage of Dragons, Ice, or Sickness, which is thought to be largely due to the lack of any human life near the corpses of these Titans.