Dungeon of Horror
The Dungeon of Horror lies at the foot of the Brogendenstein Mountains, to the north of the Barbarian Camp. Or rather, it's smoking ruins do, as it's no longer a place anyone can actively explore, unless you enjoy looking at piles of rubble.
The Dungeon of Horror used to be home to a Gate to one of the Nine Hells, and was overrun by Hellspawn Warriors and Mages, ruled by an Overlord. Its only real main point of interest was a portal into the Dark Spires, and so it was rarely traversed, as its dangerous nature was well known to any who travelled the area.
It was later destroyed by an insane creation of Kraven's, a massive Bone Golem created from the body of a Fire Giant King. The entire castle was plummeted into the ground, and the Gate was sealed shut, ensuring that no further spawn of Baator would be able to get into the Prime from that location.
Today, all that remains is a smoky ruin, and if one looks hard enough, you can find a tunnel that goes from the bottom of the mountain, directly into the top floor of the Dark Spires. The place is rumored to be cursed, so even with this shortcut, it still doesn't see much use by adventurers in the area.