From The Elder Scrolls - Wiki on Neoseeker

Summitmist Manor

Quest Giver: Ocheeva in Cheydinhal
Previous Quest: Bad Medicine
Next Quest: Permanent Retirement
Reward: Leveled gold (600 max), Night Mother's Blessing
Fame/Infamy Change: Infamy +1
Quest ID: Dark08whodunit

Walkthrough

The Night Mother's Child

Talk to Ocheeva and accept the next contract. A man is throwing a party in Skingrad at Summitmist Manor and has sent the Dark Brotherhood an invitation. You are to go to Summitmist Manor and speak with Fafnir, the "doorman" (actually a Dark Brotherhood agent). He'll lock you in until you complete the contract. There are five guests:

Matilde Petit - A racist and arrogant Breton
Primo Antonius - A rich and arrogant Imperial
Nels the Naughty - A crude, drunkard Nord
Dovesi Dran - A shy and shallow Dunmer
Neville - A strict and abrasive Redguard

All of these people have wronged their host in the past and he wants them dead. The condition for this contract's bonus is straightforward: don't let anybody know you are the killer.

Open Your Eyes

Killing all of the guests is easy, they all have one hit point; a single punch is enough to kill any one of them.

Fulfilling the bonus is no more difficult. If you kill a guest in front of another guest they don't notice. The only way to fail the bonus is to use offensive magic. If you use any offensive magic, even when you and a victim are alone in a cell, everybody else will know about it. When there is only one guest left and if he/she attacks you this does not count as detection, you will still get the bonus. There are different dialogue options with different guests and certain ones are only available if your disposition is high enough. You can trick them into waiting in certain locations so you can kill them alone but there is no purpose. As soon as you walk through the door and your conversation with Matilde is complete, kill her with a single punch. Now run around the whole mansion and punch everybody else dead. The process shouldn't take any longer than 30-45 seconds. Exit the front door and return to Ocheeva for your reward and bonus.

Reward

For pulling off the easiest contract in history you will be rewarded with leveled gold to a maximum of 600 and the bonus of the Night Mother's Blessing. The Night Mother's Blessing confers a permanent +2 bonus to your Security, Marksman, Sneak, Blade, and Acrobatics skills.