Ascension Quest: The shadow lord class is only available in the Collector's Edition. To become a shadow lord, you must first defeat Nauselom in the Shadow Ruins. Then when you return to Staroxia in the Forbidden Lands, she should give you a new dialogue option about the shadow lord amulet that you found on Nauselom's corpse. When you ask her about the amulet, she'll offer you the ascension quest, provided that you have a third tier class slot available.
Note: If Staroxia doesn't offer you the quest, be sure that you've identified the amulet. You might also need to view your quest log to update its status.
The ascension quest involves travelling back to the Isle of the Damned (where you picked up the shadow crystal needed for entry into the Shadow Ruins) and there defeat a dracolich. Then when you return to Staroxia, she'll reward you with the shadow lord class.