These gargantuan spider-creatures can be found in the Underworld as part of several quests, where they have to be killed to restore the monuments of Grenth in particular (but not exclusively). They are the main brunt of Dhuum's forces.
When attacking them watch out for the Meteor Showers they like to cast at the beginning of the fight. Spread out and hang back until they have cast them, then engage. Also, if possible, stay spread out during the fight, so that their Fireballs are less effective.