Ylem Grav is an ancient God who's name has been forgotten over the passage of time. As his power waned, he found his very existence being threatened. In order to prevent himself from ceasing to exist, he used what remained of his power to craft a pocket universe that was connected to a cursed magical artifact (the cover illustration denotes a sword, but the artifact can take whatever form you need it to). Those that come in contact with the artifact are transported to his dimensional realm, where their presence increases his strength, and sustains his existence. Those that seek to leave are slain and their souls enslaved in order to become one more barrier between his prison and the freedom of any that find themselves trapped within its confines.
Through whatever means, one of the party members has got their hands on that cursed artifact, and now they must fight their way through the trials in order to slay all of the souls that are giving power to Ylem Grav. Once the souls have been destroyed, Ylem Grav will be weak enough for the players to defeat and end his blighted existence once and for all.