Soul cairn is accessed by portal in castle Volkihar ( not sure of spelling

) and only activates once you reach that part of the quest... you get a few new caves, like the one leading to the area fort dawnguard is in...the forgotten vale is a new area far to the north-north west...about the size, maybe a tad larger than blackreach...a lil advice...the Soul cairn requires a little patience compared to everywhere else in Skyrim, but is worth it.