Scheduled for Execution
Oblivion: Dark Brotherhood Quest
Valen Dreth. If your memory goes back far enough, you'll remember this guy from the very beginning of your game. He was your neighbour in the Imerial Prison, taunting you from his cell.
Now, he has to die.
You'll need to retrace your steps from your original escape from the Imperial sewers. Vicente gives you a key to open the gate into the sewers. From there, follow the map markers until you reach the prison.
As long as Valen Dreth dies, the contract is fulfilled. However, if you want to earn your bonus, you need to sneak your way in. If any Imperial Guards die in the process, your bonus is lost.
The best way to get in undetected is to combine a reasonable Sneak skill with an Invisibility spell or potions. You can buy the potions in various locations, or get the spell from a merchant in the Dark Brotherhood sanctuary.
When you finally reach your former cell, you'll need to wait. Valen and a guard are having an interesting conversation about his upcoming release. Once the guard walks away, you'll have the opportunity to strike. You can speak to Valen if you wish. He's easy to kill, and with a ranged spell or arrows, you don't need to open his cell.
Vicente will give you a gold reward, and if you fulfilled the bonus, the Scales of Pitiless Justice (which gives you a +2 bonus to strength, intelligence and agility. Next, you move on to The Assassinated Man.