Kaalgrontiid (Dark Aeon)

Kaalgrontiid is a colossal green dragon who was imprisoned by Khunzar-ri in the Halls of Colossus long ago. Locked behind a magically sealed door at the Summit of Rending Flames, Captain Calsivius attempts to break the seal to control him for Euraxia Tharn, and after he is killed, Abnur Tharn combines the Wrathstone to undo the spell. However, the door is opened, and Kaalgrontiid emerges from the depths before flying off into Northern Elsweyr.

Source: uesp.net.