The Schism, which occured many years before the start of the Door Within Trilogy, was caused by the spilling of innocent blood.
Glimpses live in the Realm, and People live on Earth. The bridge between the two is called The Thread. Many, many years ago, though, the Realm and Earth were one. All was perfect—all was well. But one day, King Eliam rode off on a mission, and left Torin, his most faithful servant, in his stead. The King warned Torin not to open the door.
But one evening, late into the night, the door was closed as usual. Torin heard scratching around it. He went to the door. "Who is it?" He bellowed. The Wyrm Lord, the firstborn of all dragons, was there. "Open the door," he whispered. Torin withheld for some time, but finally gave in. He opened the door—and all were killed.
The Realm was divided at this point, in The Schism. Innocent blood had been spilt. However, the story of the Great Betrayal was passed down through the generations in both worlds, so all knew the story—but the question was whether or not they would believe.