Cataclysm is a mod that adds difficult dungeons, challenging boss-fights and powerful items. It is inspired by MC Dungeons. There is an unofficial wiki which is mostly comprehensive and can help guide you through the bosses.
This page will detail some of the bosses as well as the balance changes made to their items.
BY DEFAULT, SANDSTORM IN A BOTTLE IS BOUND TO X! (Cataclysm Action Key)