Complex sentence: The Empire of Thyatis is a powerful state in the Dungeons and Dragons Mystara setting, first mentioned in Module X1, Isle of Dread, which describes it briefly.
Split into simpler sentences: The Empire of Thyatis is a powerful state in the Mystara campaign setting for the "Dungeons and Dragons" fantasy role-playing game. Thyatis was first mentioned in Module X1, Isle of Dread, which describes it briefly.