Oblivion:Scheduled for Execution

Quick Walkthrough
# Take the key from Valtieri, and make your way through the sewers.
# Find Valen Dreth in his cell, and kill him.
# (Optional) Do not kill any guards.
# Exit the prison and return to Valtieri.

Detailed Walkthrough
No Rest for the Wicked
The mini-quest to get the contract from Vicente Valtieri is named "No Rest for the Wicked". Vicente will tell you your next target is Valen Dreth. The name may sound familiar—he was the Dark Elf taunting you inside the Imperial Dungeon at the very beginning of the game. Revenge is a dish best served cold.

Getting through the Sewers
You return to the Imperial dungeon basically the same way that you got out in the tutorial: through the Imperial Prison Sewer to the northeast of the Imperial City. Valtieri gives you the key, which is necessary because the sewer was sealed shut following your escape. Have a few lockpicks handy as there are a couple gates that do not open with the sewer key. Early on there is a detour through a couple of sewer sections that you did not previously visit, but then you'll proceed through the same Sanctum and Imperial Subterrane as before. If in doubt, follow your map marker.

Caution is needed in the Sanctum and in the first part of the Imperial Subterrane: guards have been posted to prevent any more prisoners from escaping. All will attack you on sight. To get the bonus you need to be sure to not kill any of them (Detect Life spells or enchantments are useful). Pause for a minute when you first enter the Sanctum to hear the guards' humorous complaints about this new duty. By keeping to the shadows and waiting patiently for the guards to move along it is possible to sneak past all of them. The most attentive guard is the one when you enter the Imperial Subterrane; he is the only one who is carrying a torch and is more actively patrolling.

If you're interested, take a slight detour into the room where the emperor was actually killed. You will find a small shrine to the emperor (including items that can be stolen), a dead Mythic Dawn agent with some notes about the investigation, and the guard leading the investigation, Captain Gepard Montrose. Luckily, the captain is too immersed in his own thoughts to notice an assassin sneaking around right behind him. In fact, it is very possible to pickpocket him and acquire all the keys for the prison, but it is not necessary, because they are easily attainable in the next part of the quest.

Assassination
Dreth will be in the same cell as before, across the hall from your former cell. When you first get to the cells, stop and patiently wait for the talkative guard chatting with Valen to leave. If he does not leave, it is possible to shoot Dreth with a bow and arrow from a distance without the guard seeing you.

If you pickpocketed Captain Montrose, you already have the key to Dreth's cell. Otherwise, another set of keys are sitting on a small table to the left of the cell. The lock can also be picked. However, once you open his cell be sure to kill him quickly (if possible, with one hit), otherwise he will take off running through the sewers, which leads to a long annoying chase. The other option is to kill him using a bow or destruction magic, in which case you do not need to open the cell door.

To get back out, you have two options. You can stealthily return back through the sewer system. Or, using the Imperial Prison Key, you can go up the stairs and leave through the front door. Once you get into the main prison area you are in a public area; being spotted here won't cause you to be arrested or forfeit your bonus.

Payoff
Return to Vicente for a level dependent gold reward (up to 600 gold). If no guards died, you will also receive the bonus Scales of Pitiless Justice, a miscellaneous item that increases your Strength, Agility, and Intelligence attributes by 2 but also reduces your Personality by 2 whenever it is in your inventory.

Notes
Upon completion of this quest, you will be eligible to do the non-contract Dark Brotherhood quest, The Renegade Shadowscale, for Teinaava if his disposition towards you is greater than 70.
If you open his cell door before talking to him, he won't run or exit the cell, and will ask you to open the door (which is already open).
If you don't feel like sneaking around and you are fast enough or have enough armor, you can just run to the cell with the guards chasing after you. You could also cast Paralyze spells on the guards, or hit them with weapons poisoned with paralyze; thus making them unable to follow you. Either way, as long as you didn't kill a guard, you get the bonus.
You cannot access Valen Dreth's cell by getting arrested. The cell you're sent to is a different interior cell than the one that you get from accessing the sewers.
Valen lacks any eating AI package, so it is not possible to kill him with a poisoned apple.

Bugs
 
You can save the game, then load it right away and the guard will walk away.

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