Diablo/Chamber of Bone
At Catacombs level 6, find and read the Mythical Book. That will talk about the Chamber of Bone and its minions, death and other things. Reading the book will open the entrance to said chamber.
Find the entrance here at level 6, and enter it if you want to. Firstly, there is nothing important here. It's just a chamber with some demons (especially skeletons) and rewards. Only good for experience and profits. Take caution when opening the door to the central chamber, as the amount of skeletons you'll have to face down is massive. If you've got a good weapon and plenty of healing potions, you shouldn't have too much to worry about. Just keep pounding away at the mob of enemies, trying to draw them through the central chamber doorway.
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.
At the end of this chamber, you'll find a book that will guarantee you the level 1 spell: Guardian.