Almost everyone has heard of Angels and Demons. Despite the ancient Pact, which suggested that they would have to always get along, they are just too different. Their wars shake the Earth, rattle the stars of Heaven and boil the magma hidden within Hell. And, unless the Inbetween step in, the Divine and Cursed beings will bring us to the end of the world.
The Inbetween are the peace keepers among these beings. Where Angels are pure good and Demons ultimate evil, an untainted Inbetween has no choice but to be neutral. When tainted, an Inbetween is a deadly force for whichever side they have joined. However, the only way to taint one is to either have them gaze upon a Divine being or have them meet the eyes of a heavily-tainted mortal.
Once, long ago, there was the War of the Angels. This was an Earth-shaking event, though it didn't actually happen on the Earth. The War of Angels, which was Angels versus Demons, occurred in the Inbetween. The Inbetween is somewhere in the middle, roughly