Catacombs added 5 new classes (Heretic; Vampiir, Bainshee; Warlock, Valkyrie), instanced dungeons (where players entering certain areas get their own private dungeon to hunt in), and new zones and quests with an emphasis on faster and easier leveling. Also includes new player model graphics and new graphics for all the games' dungeons (except Darkness Falls). As of February 6, 2007, players may now download and venture into the depths of the Catacombs for free. This expansion can be downloaded via the website for no extra charge.
Source: Wikipedia, "Dark Age of Camelot", available under the CC-BY-SA License.