From publisher blurb:
Several centuries past, a shadowy sect sought to enslave the world in the name of their dark lord, Baalzebul, one of the arch devils of Hell. They carved out a massive subterranean temple deep in the heart of a dense, forbidding forest to base their operations. The clerics of Tyr, God of War and Justice, were eventually able to assemble a host of holy warriors to raid the temple, and managed to destroy most of the wicked followers of Lord Baalzebul before their darksome ambitions could be fully realized. Or so they thought. Cut to the present: A company of clerics calling themselves the Order of the Black Lotus have journeyed to the evil fane with the intent of reviving the forgotten cult of Baalzebul. They are surprised to discover that many of the followers of the arch devil survived the raid, afterwards performing the necessary rites to merge with the Negative Material Plane so as to unnaturally extend their lifespans. It is up to you to see that they don't fully unite with the undead servants of Baalzebul, unleashing the arch devil's horde upon an unsuspecting world.
This product is designed for 4-6 player characters of levels 13-15, and contains a detailed key and maps to the sprawling three level underground Temple of Baalzebul. Also includes a new spell, a new monster, and rules for the worship of the arch devil Baalzebul.