User blog comment:JAlbor/Wikia at E3 2012/@comment-5018658-20120525125513/@comment-4772788-20120527201953

Also Nocturnal did have a vessel. The Ebonmere, which is why you need to restore the ebonmere to be able to recieve Nocturnals blessing, because her influence was severely weakened by the theft of the skeleton key. The skeleton Key acted as a conduit for which Nocturnal could appear on Tamriel, basically this means she has a permanant route from Oblivion to Tamriel