Beyond the Door (2009)

From the front of the book:

In a dark corner of the elemental chaos a strange, stone obelisk marks the location of a plain, iron trap door. No lock, ward, or curse guards this portal, yet none who have passed through it have returned to speak of what rests beyond. Some claim that the mortal corpse of Vecna, guarded by undead horrors beyond reckoning, rots away beyond it. Others whisper that the door leads to the black heart of the Chained God, the last physical shard of that dread being. The advice of the wisest sages has led you to this place. You seek the wand of Tzunk. Every sage agrees that the wand was last seen in the hands of Darvek Hahn, and Darvek was last seen passing through the door before you. Can you avoid whatever terrors have claimed so many lives? An adventure for 18th-level characters.