Oblivion:Unfriendly Competition

Quick Walkthrough
# Learn of the unfriendly competition.
# Speak with Thoronir and learn about his inventory. When he leaves, follow him.
# Eavesdrop on the conversation then follow Agarmir to his house.
# Find the Macabre Manifest in the basement and confront Thoronir with it.
# Head to the Mausoleum and deal with "the competition."
# Go back to Thoronir with final proof of Agarmir's deeds and collect your reward, then visit Jensine for an additional amount of gold.

Detailed Walkthrough
The Merchants Problem
At the Imperial City Market District speak to some of the merchants. Upon speaking to the merchants (either Tertullian Verus, Claudette Perrick or Ogier Georick) you will learn of a new merchant in town, Thoronir, whom the other merchants don't trust. They will tell you to speak to Jensine for more information.

Unfriendly Competition
Speak with Jensine at Jensine's "Good as New" Merchandise in the Imperial City Market District. You will learn that Thoronir is selling goods with an incredibly low markup price compared to other merchants, essentially committing price dumping. Volunteer to help the Society of Concerned Merchants deal with 'Unfriendly Competition' from Thoronir at The Copious Coinpurse.

Eavesdrop on a Conversation
Speak with Thoronir in the Copious Coinpurse and learn about his inventory. Wait outside the shop until after 8pm, and then follow Thoronir. He will either wander the city or go drinking in The Merchants Inn for several hours - before eventually making his way to a small garden in the Market District around midnight. Agarmir slips into the garden soon after. Sneak close enough, and you will enter a cutscene. When they have finished talking, follow Argamir back to his house in the Talos Plaza District.

Agarmir's House
Agarmir will lead you to his house, but you will need to wait until he leaves it again to enter and find evidence. He leaves at different times depending upon the day of the week: on the weekend (Loredas and Sundas) he is out from 8am until 8pm; from Morndas to Turdas he leaves just after 4pm and returns after 8pm, and on Fredas he leaves at 5pm for two hours. Once he leaves, pick the lock or pickpocket Agarmir for the key to gain entry to the house. There is another lock to pick to get into the basement, so make sure you have plenty of lockpicks. Take the book called "Macabre Manifest". It will be on a table in the basement surrounded by candles. Go back to Thoronir and confront him with the book. He'll claim he didn't know the goods coming from Agarmir were looted from corpses (hence their cheap price) and want to make amends and make sure Agarmir gets caught, and will suggest you go to the Trentius' Family Mausoleum to gather more evidence.

Mausoleum
Go to the Mausoleum, which is among the gravestones in the Palace District, and confront Agarmir. Be very careful not to attack Agarmir or his hireling, Rolgarel, until they have first attacked you. Sneak attacks (or any other crimes) will lead to you getting a bounty, and are very likely to trigger a bug that makes it impossible to finish this quest.

Once you have killed them both, collect any loot in the mausoleum. To get out of the mausoleum you need to get the Trentius Mausoleum Key from Agarmir's corpse. To complete the quest, you need to be sure to take the muddy shovel as evidence. In addition, there is some valuable loot (that can be more worthwhile than the quest's official rewards). Agarmir carries a leveled, unique, enchanted sword, Debaser, which drains Willpower and Endurance. Hidden behind the grave of Calliben Trentius is Calliben's Grim Retort, one of the few magic items with guaranteed locations. To reach the mace, jump nearby and grab it as you fall back to the ground, or use the  weak fireball or similar spell to shift it into a position where you can reach it.

Reward
Return to Thoronir with the evidence and he will tell you he has made amends and joined the Society. He will also reward you with the Weatherward Circlet, a leveled ring with Resist Frost and Resist Fire enchantments. Lastly, return to Jensine for a leveled gold reward. When talking to Jensine, you need to choose the topic "Thoronir"; this topic can often be too far down Jensine's list of topics to be apparent when you first start talking to Jensine. Be sure to use the scroll bar to scroll through Jensine's complete list of available topics.



Notes
If you're having trouble activating the "Follow Thoronir" part of the quest, keep in mind that it doesn't matter too much if you're seen. Simply run around the small courtyard and eventually your character will lock in place and Agarmir and Thoronir's conversation will begin. The activation spot appears to be centered around a pair of small bushes nearby.
It isn't necessary to follow Thoronir all over. You can shortcut by waiting and being at the meeting point of the courtyard around midnight.
If you pickpocket Agarmir's Debaser before going to the Mausoleum, he will not have it during the confrontation in the Mausoleum. He may acquire another weapon before the fight; otherwise, he will either attack you with his fists or grab Calliben's Grim Retort when the fight starts (unless you can take it first, in which case he will resort to Hand to Hand combat).
Successfully casting a high-level Frenzy spell on Rolgarel before Agarmir initiates conversation with you will result in the two fighting each other. Thoronir's and Jensine's rewards can then be collected as usual, and the quest completed, without issue. If he's still alive, Rolgarel will remain in the Trentius Family Mausoleum indefinitely as a non-hostile NPC.
You don't actually have to follow Agarmir. Simply walking near his house will trigger the quest update.
Despite Thoronir saying he gave all of his old inventory to the temple, he will have the exact same items for sale as before.
In the construction set, the script attached to Agarmir's Shovel has a note that says "sets stage on sword pickup", hinting that originally picking up Agarmir's Debaser would progress the quest instead of the shovel.

Bugs
 
 
 
 
 
 
 This can be fixed by using the console. Click on the front door to the Mausoleum, then type  .
If you can cast a strong Charm spell on the guard when he first appears, he will "pay the fine" for you, and you will remain inside the crypt. If you talk to Agarmir before the guard arrives he will become hostile, and the guard will help you fight him.
 
 
 This can be fixed by using the console. Before entering the tomb type  .

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