In desperation the Will of the humans on Rift World pleaded to reconnect with their kin across the stars, but they made a mistake.
In an attempt to reestablish the connection of Will with neighboring worlds an experiment was preformed to try and channel the collective will of the whole world together into one psion who would open a gateway to the other world. Unfortunately psionic astral gates and human hive minds don't work out well. The energy was channeled through an observatory and the collective power was enough to rip a whole in local space, but it was no portal, and the force of it destroyed the observatory and the psion sending a backlash across the humans living in the world driving them mad. At first the Rift was only a few millimeters in length, but periodically it expanded and unleashed dangerous astral radiation.