The Great Prison

Deep within the heart of Bluewood lies the Great Prison. Beneath the heretical college of Deil Tor, those ambitious warlocks have constructed their palace of ill-gotten knowledge over a dynamo of mysterious and bizarre energies. The Great Prison, by all records and knowledge, was present when the first elven scouts explored the depths of Bluewood.

Theories

 * Warlocks did not go to the Great Prison.  Law-abiding wizards traveled to it and were turned by the dark will lurking inside.
 * The Great Prison is flawed, and the necromantic energy that pervades Bluewood is brought upon by the cracks in the prison.
 * Bluewood itself is part of the prison, and the necromantic energy is a tool to keep whatever is captured locked away from the world.
 * The Great Prison does not in fact contain anything, and if it ever did, it has been destroyed long ago.
 * Not one, but two entities are trapped within the Great Prison.  A foul demon does eternal battle with a holy angel, and the prison was built to protect the world from their conflict.
 * The Great Prison is a repository of all the ancient evils that wandered the world when it was first brought into reality by the Great Four.  Captured by the Pentarchy as they traveled the globe, they constructed the prison as a place to store the evils they could not destroy.