3: Accidents Happen

The Elder Scrolls IV: Oblivion Guide - Dark Brotherhood

3: Accidents Happen

A Wood Elf named Baenlin is your next target. He resides in Bruma, and is apparently quite wealthy. To receive a bonus reward upon completing this quest, you'll have to kill Baenlin in the fashion described by Vicente: the murder must look like an accident, and Baenlin's servant must not be killed.

Vicente explains that Baenlin sits in his chair from 8:00PM to 11:00PM each night. Above his chair is a Minotaur head, mounted to the wall by iron fastenings. A section of the wall on the second floor of Baenlin's house can be removed to enter some crawlspace. Once in the crawlspace, you must loosen the fastenings while Baenlin is seated in his chair, causing the trophy to fall on him. Do this, and Baenlin's murder will look like a horrible accident.

Travel Bruma and locate Baenlin's house. Don't enter by the front door; instead, walk around the house and go through the door to Baenlin's basement. It is best to wait until around 9:00PM before entering his home to carry out the contract, since Baenlin will be seated in his chair from 8:00PM to 11:00PM. When you enter at this time, Baenlin will be out of your way, allowing you to climb up the stairs and enter the crawlspace unhindered.

Sneak into Baenlin's house by picking the lock on the cellar door behind his house.

Wait until the time is around 9:00PM, then approach the door to Baenlin's basement while in Sneak Mode, and pick the lock. Only attempt to pick the lock while the eye icon is faded, as this indicates that you are not being watched. If the icon is bright and solid, wait in Sneak Mode by the door until the icon becomes faded before attempting to pick the lock. If you enter the house and your target is not sitting in the chair, once in the crawlspace, you can use the wait feature to pass the hours ahead to sometime between 8:00PM and 11:00PM.

Upon exiting the basement to the main floor, head up the steps straight ahead and enter the room at the top. Open the crawlspace door in this room and go inside. Approach the fastenings and loosen them while your target is sitting in his chair at any point between 8:00PM and 11:00PM.
When the deed is done, very slowly climb back down the stairs. Gromm, Baenlin's servant, will be standing over his master's dead body. Creep down the stairs, enter the basement, and exit the house. If Gromm does spot you, he will follow you outside, and the city guards will likely chase after and kill him. If this happens, you'll lose Vicente's bonus reward, so take your time.

Activate the fastenings at the back of the crawlspace on the second floor of Baenlin's house between 9:00PM and 11:00PM to make the murder look like an accident.

Once home free, return to the Dark Brotherhood sanctuary in Cheydinhal and speak to Vicente there. If you met his conditions, you'll receive the Sufferthorn blade as reward. Either way, you'll be promoted to the Slayer rank. Accept another quest from Vicente to complete 'No Rest for the Wicked.'

Making the killing look like an accident.