Delved deep into the Dourstone mine. After exploring the mine (left. right, right, left through barricaded tunnel), discovered the elevator that led to the Dark Cathedral chamber, from which three halls branched. They decided to explore the eastern hall, after dispatching some evil-aligned planar-touched creatures that fled in that direction.
It turns out that it is a temple dedicated to Hextor, the Scourge of Battle. The party successfully fought their way through the entire complex, essentially slaughtering every last cultist, save one, the apparent leader who very bravely fled the complex while his minions stayed to defend it!