Diablo/Chamber of Bone

Catacombs level 6, find and read the Mythical Book. That will talk about the Chamber of Bone and its minions, death and other things.

Find the entrance here at level 6, and enter it if you want to. Firstly, there is nothing that important here. It's just a chamber with some demons (specially squeletons) and rewards. Only good for experience and profits.

Go explore the chamber and activate the levers to open secret rooms. Kill the monsters and get your rewards. Luckily you may find some unique or magical items that you can use or sell for money.