It states that Valerica locked Serana in a tomb for decades. This time period has to be much longer considering Serana is suprised to find there is an Empire based in Cyrodill which means she was sealed away before the Empire existed.
EXACTLY WHAT I THOUGHT. LOL
After completing dawnguard by killing Harkon with the vampire hunters, I tried to go to the soul carin to tell Valerica that Harkon was dead. The game would freeze every time I approached the keep on the 360. 220.127.116.11 03:51, March 13, 2014 (UTC)Kuronin