This one was originally conceived for Eberron, but I think it could be revised to fit another world.
Dream-beings are invading from the plane of dreams (if you're using Eberron, this is the Kalashtar/Inspired), but it is a silent invasion. Rather than entering the material plane physically, they bond to human hosts that they have manipulated through dreams and have, over the course of centuries, built an empire.
There are a few of these beings who rebelled against this evil, and bonded permanently with a human host, escaping the realm of dreams to help defend the world. For millenia they have been locked in an imperceptible battle for the future.
What few even of these beings know is that their projection into the material plane is secured through one being: the Dreamer. Many eons ago, the dream-beings invaded, and were turned back by the ancient Giant empire. The giant's used incredible magic to prevent further incursions from the plane of dreams.
But, in the aftermath of the invasion, many beings were touched by the power of the dream-beings. The Dreamer was one. He wielded the incredible power to project his dreams onto reality (see the Awakened Dreamer in Hyperconscious from Malhavoc). His dreams became nightmares, and he terrorized the world until a cryomancer (Frostburn) and a Great White Wyrm joined to imprison him in the Tomb of Frozen Dreams (Complete Book of Eldritch Might from Malhavoc). The dragon perished in the deed, but the cryomancer lives on guarding the tomb where the Dreamer dreams eternally in the ice.
But the Dreamer is not dead, and his dream continues. And it is through his dream that the dream-beings can gain influence in the material world.
As the PCs become involved in the conflict, they eventually discover the secret of the Dreamer and come to the Tomb. However, the Cryomancer knows something they don't: because the evil dream-beings are only temporarily bonded to their hosts, they will be expelled but not destroyed if the Dreamer dies. However, the good dream-beings are permanently bonded and will be destroyed if the Dreamer dies. And releasing a being as powerful as the Dreamer is a risky prospect at best...