Black sand dealing negative energy damage.
Rest of Vecna's hut dealing massive negative energy damage.
Black sand is a type of magically-tainted sand found in the part of Urruk that once contained Vecna's sanctum, before he ascended to the astral sea to contend with Ioun for The Source. It deals a small amount of negative energy damage to those who touch it, and also contains a small army of animated skeletons.
Draven took a sample of it after the party encountered the site and Quill18 has intimated that there will be an important payoff from this event.