Whodunit?
Oblivion: Dark Brotherhood Quest
This is one of the most fun quests in the game. You're going to a party, and you're going to kill everyone there.
Head to Skingrad and talk to the man outside Summitmist Manor. He'll let you know that all of the other five guests are inside. They've been told that there's gold hidden in the manor, and that they're all locked in until someone finds it.
Of course, there's two good ways to do this. The first is to unleash the fury the second you come in. That'll end this pretty fast, and although it's fun to watch them run, you miss out on the detail of this quest, as well as your bonus.
Instead, you might have more fun playing it out. The goal here is to kill all five of them, but one by one in isolation. No other party member can see you commit a murder. You'll want to talk to the guests, and raise your disposition with them to be as tricky as possible.
The first guest (Matilde Petit) who speaks to you upon entering, for example, can easily be tricked into forming an alliance with you. You can send her to the basement to search.
The other guests can be tricked in similar ways, and even without talking to them most will wander into the upstairs bedrooms. It's not too difficult to complete this one with the bonus intact.
Return to Ocheeva and you'll receive your gold reward, as well as the Night Mother's Blessing. This bonus is a permenant +2 increase to your Sneak, Security, Blade, Marksman and Acrobatics skills — nothing to frown at.
Speak with Ocheeva again to move on to the Permanent Retirement quest.