This underwater hideout provides your infamous character with safe haven. Those willingly inflicted (or shall we say graced?) with vampirism will find a means to feed, and if necessary, a means to cure their infliction. Hire evil minions, pray to the dark god Sithis, and a host of other functions await.
This new content will be available for $1.99 (which is a dollar cheaper than Mehunes' Razor) and will weigh in around at 7.0 MB. You could get it from Oblivion's downloadable content page, when it's out tomorrow.