Lair of Infinite Agony is a new zone in Tiny Tina's Assault on Dragon Keep DLC
You will find a good horde of Skeletons waiting for you once you cross the threshold into this area. Be sure to look up and take out the Skeleton Archers that are very likely on the ledge above you.
Accessing Next Area
Head over to the West to make your way toward the Grim Stone Sepulcher As you make it to the Western area you will find yourself facing a trap. This one is all about timing to make it through the raising and falling floor plates. Once you make it to the other side, and it can take a few tries, you can throw the switch there and stop them from moving.