Another mission that gets you a new kind of lair to live in. This time you'll get a journal entry saying that you've inherited a place called Deepscorn Hollow. Go there and dive when you're near the map marker. You'll see a sunken trunk - the entrance is inside it. Read the journal you'll find inside, and you'll be told to pay a visit to Rowley Eardwulf in Wawnet Inn (near the Imperial City) to get some additional equipment. As always, you'll have to buy all the expansions in order for the quest to end.
Rowley's "store" is only open at night - when you talk to him during the day, you won't be able to trade with him. The mission is over when you buy all the furniture elements for your evil lair. Besides them, also buy the Ichor of Sithis. When you're back to Deepscorn, use it on the Sithis' altar to create an evil version of the shrines you find in Chapels throughout Cyrodiil. You can use it to heal diseases and lift curses.
Our vampire minion.
A deluxe coffin = the quickest way to vampirism.
As a ruler of such an evil and dark place as Deepscorn, you have a handful of things to entertain yourself with. The coolest thing is you can instantly become a vampire and cure vampirism at any time. To become one, just have a sleep in a decorated coffin in one of the rooms. To lift the curse, take some Purgeblood Salts from the vein on the wall (screenshot below) and touch the pillar in the middle of the "fountain" room. It's really that simple now.
Purgeblood Salts, a perfect vampire cure.
An altar of Sithis - comfy.
Apart from all that, you'll also find a botanical garden here, filled with all sorts of poisonous plants, a prisoner in an eternal coma (you can use it to feed as a vampire), and a vampire minion whom you can send to assassin somebody and bring you the loot.