This is the final part of the Dark Brotherhood Quest line. In it you refurnish the Sanctuary in Dawnstar. It isn’t very difficult, you just visit the Thieves Guild in Riften.
I found that all the upgrades except the garden are a waste of money because they are visual rather than practical. Although it does make the Dawnstar sanctuary look a lot better.
You can keep doing Dark Brotherhood quests for quite a while, just walk up to the Night Mother and she will give you an assassination assignment. These aren’t fun but are a good way to make a little money on the side.