The Lonely Realm

From Introduction:

For hundreds of years academy wizards tended to the Realm of Arden. Originally only a small glade formed from the Void, the wizards nurtured and grew it until Arden stretched 150 miles across. The secret of the pocket world could not last though and twenty five years ago the Lich Rezial seized control of the tower at the heart of Arden, sealing the gate to the world the wizards and their priestly allies called home. Still, the lich fears that adventurers or heroes from beyond might find some other way to enter his personal dominion.