The Sinister Cavern' is a location in Orobas Fjords up a waterfall at the end of the little river that makes it. Inside you will find abominations and reach a cavernous area of lava with some nest towers spawning enemies and a necromancer named Barnabus on a shielded platform. You take your dragon form to get around and take teleporters to rooms with more abominations and shield generators that are shut off via lever. One of these areas (there are three) contains the Book of the Dead needed to complete the Book of the Dead quest given by Igor at your Battle Tower (if you chose him). Once you shut them all off, you can face Barnabus who will also summon an abomination to help him and he will heal it on occasion. Kill him and you will get his mace which is proof of his death for the A Hunting We Shall Go Again quest as he is on the wanted list, and you get his ring which you get to keep. He will also drop Razakel's Hammer if you had used mindread on Razakel back when you were getting the Battle Tower from Laiken.
The book of the dead is found in the area which the teleporter highest up, near a dragon skeleton, teleports you to.
A dragon skill book can be found on the table next to Barnabus.
Additionally, the Aleroth Archmage Necklace can be found within a locked chest in one of the caverns that holds a machine generating the barrier protecting one of the nests. The key to this chest is located within the flames of the flaming "window" where the three halls meet to make the T junction in the cavern containing the jumping puzzle/platforms.