Premise: The local House Hlaalu official is concerned by the disappearance of one of their affiliated outlander merchants and asks the player to investigate. The merchant turns out to have been murdered. The player is framed for this and must clear their name. They can find clues scattered about Andothren and in a nearby cave that points to either House Indoril, the Camonna Tong, or the Sixth House cult as being the actual murderer.
outlander merchant’s residence interior
smuggler/Sixth House cave dungeon
letter addressed to nephew
threatening letters addressed to merchant
Shady Dunmer NPC
House Hlaalu questgiver/steward in Andothren (if none existing)
The local House Hlaalu official is concerned about the disappearance of one of their affiliated outlander merchants. The player first is sent to investigate the outlander’s house. As evidenced by the stashes of ash statues and other illegal goods (skooma, dwemer artifacts, etc) hidden in their residence, it becomes clear the outlander was a smuggler along with being a Hlaalu merchant. (The interior dev is encouraged to get creative with the placement of these illegal goods, such as behind false walls or hidden behind furniture, etc.)
There also several threatening letters found stacked on the outlander’s desk. These letters are unsigned, and demand the outlander to either let the writer in on the profits, or to clear out of town. The player can gather up these papers to report in to the House Hlaalu official.
When the player returns to the Hlaalu official, the House Hlaalu official is speaking with a shady looking Dunmer. The official turns to the player to say they are sorry, but the player has been accused of murdering the outlander merchant. The official goes on to say the player have been given a chance to clear their name, but that the House cannot offer them anymore assistance until they do. The shady looking Dunmer doesn’t speak to the player, beyond saying something racist before forcing a Goodbye.
The player is then left to keep searching for evidence to clear their name. There are a few trails to follow:
 There are two ways to get to this lead. One is rumors about the town saying the merchant’s nephew is visiting to check on his uncle, who he is worried is getting involved in the bad parts of Morrowind society. Two is, with a high enough disposition, the Hlaalu official suggests the player should go have a drink in the tavern, but does not explain why.
If the player goes to the tavern, the tavern owner says someone has been looking for them and nods them over to a patron drinking in the corner. This patron is the nephew, and if questioned, shows the player a letter his uncle had sent him a few weeks prior. The letter details how the outlander tried to broker a trade deal with House Indoril, by giving them a cut of the profits if they helped protect the outlander’s smuggling operation from “the Dunmer natives”. The letter ends saying the merchant will travel to a cave specified by their contact to work out the deal, and contains directions to the cave. If asked about anyof this, the nephew says he was too frightened to investigate much himself, and asks the player to hurry and make sure their uncle is okay.
 Asking about town also points the player to a Dunmer merchant who supposedly had a partnership with the murdered oulander at one point. This Dunmer is a Indoril-aligned merchant working under the behest of Lord Ilvi. He admits it is not typical for the Indoril to engage in much trade, particularly with the Hlaalu, but that his goals are for Lord Ilvi to know only. If pressed (persuaded) by the player, he explains he had been trying to broker a trade partnership with the outlander before the outlander had disappeared, and had had to pull out of the deal due to the outlander getting involved in illegal operations. He states proudly he is loyal to Temple law, and lambasts the outlander for being a Hlaalu heretic who should never have come to the Thirr River Valley. With a little more persuasion, he would also admit the outlander merchant was trying to convince him to trade in ash statues, but that he, as a faithful followr of the Tribunal, refused of course!
 In an alley of the town is a Sleeper, who can give clues as to the merchant’s fate through his confused ramblings. The dialogue they have should be sensitive to the completion of the main quest. For instance, before the Sixth House has awoken, the Sleeper may say: “ Try... down. By the waves. In the dark. Uh, I mean you could try looking near the water. Sorry, I’m rather preoccupied, try asking someone else.”
...whereas after the Sixth House has awoken, the dialogue might be: “Dragged where surf and crawling worms go down, down, down, at behest of star-wound and the red in my eyes. Into darkness he climbed, twisting scales and bringing smiles to the sleeping, dreams to the betrayed...my dreams, oh, my dreams. Leave me, outlander!”
...while after the main quest, the Sleeper tells the player he believes the outlander was murdered in a Sixth House cave and gives them directions to it, asking the player to not tell any others of his “indiscretion” in return.
 If the player returns to the murdered outlander’s house, they find it has been gratified with terms like “heretic”, “n’wah”, and similar threats on the walls in blood. New unique ash statues can also be spotted, placed in areas like the merchant’s bed, his desk, the window sill, etc. The player can pick one of these up to return to the Hlaalu official as evidence.
The first three routes lead to a cave down by the coast. The first few rooms of the cave should look like a smuggler’s hideout. The last rooms should have open crates of ash statues scattered around (but no other overt signs of the Sixth House). The dead body of the merchant is also in here.
Next to his body is his journal. The journal details all that is happened so far: being threatened by a syndicate (he writes he thinks it’s the Camonna Tong), losing his Indoril business parter over a disagreement, writing his letter to his nephew, and then coming to the cave to meet his contact. This last entry details some confusion as to what the contact wants from him, and his hopes that the Indoril has finally seen reason.
After gathering this journal, the player is again given a chance to speak to the nephew, the Indoril merchant, and the Sleeper if they missed these steps beforehand. They may then return to the Hlaalu official, and have three options, assuming they gathered all the evidence possible:
1) Only shows if the player spoke with the Indoril merchant. They can accuse the House Indoril merchant of the murder, on the basis that he believed the outlander to be a heretic and “took care of him”. The Hlaalu says this is just another regretful symptom of the brewing conflict in the Thirr Valley.
2) Only shows if they have the nephew’s letter and the merchant’s journal. They can accuse the Camonna Tong, saying they hate outlanders and wanted to gain control of the outlander’s smuggling operation. The Hlaalu hushes them up, saying accusing the Camonna Tong is very dangerous and to say no more of it. However, they say the player has gained enough evidence to clear their own name at least, and they must continue to be vigilant against the real killer.
3) Only shows if they spoke to the Sleeper, or picked up one of the unique ash statues in the merchant’s house. They can accuse the Sixth House cult of the murder. The Hlaalu official is disturbed by this development, and asks the player to keep it quiet so they do not raise a panic.
So, this is constructed that it actually could be any of the three. My head canon is that it is a Camonna Tong agent falling to Sixth House corruption, but since this is a hot button issue, it should make sense for the Indoril merchant to be the killer, as part of the Indoril-Hlaalu conflict. The player can choose any of the three to blame and finish the quest.