The Mortal Races
First there was nothing, then there was God. He was lonely and thus created the Planes. At first, there was only the Planes of Heaven. He created his first creatures, the angels. Together, the expanded the Planes of Heaven into nine planes. Dissatisfied, God and the Angels created the Mortal Planes and filled it with every creature imaginable. The Angels and God held ultimate power over all. Then, betrayal, and God's most trusted adviser, Lucifer, fell from grace. However, Lucifer took the secrets of creation with him. With it, he created the nine Planes of Hell. Thus began the war for control and domination. The Mortal Planes became a battleground with each side finding the mortal races adept soldiers. The war devolved into a race for resources. The resources being the mortal races.
The war is on going and a resistance force has begun in The Mortal Planes. The mortal races want autonomy and freedom. They fight against the experimentation and exploitation of their fellow mortals. The experimentation has created new races. Ones that are strong enough to stand on the same field as angels and demons. The war has become a three way conflict, with the mortal races stuck between the planes of Heaven and Hell.