Board Thread:Skyrim/@comment-1157147-20130322022136/@comment-78.57.223.161-20130713160617

How about this explanation for his apearance there? His den(im refering to the places from which Alduin revived the dragons) was above Blackreach, and as Alduin was reviving him, the den collapsed down into Blackreach, with him tumbling down aswell, and then the hole left after the collapse just caved in.