Skyrim:Sanuarach Mine (quest)

Quick Walkthrough
# Speak with Ainethach about the troubles with Sanuarach Mine.
# Talk to Atar about why he shut the mine down.
# Convince Ainethach to sell the mine, or convince Atar to leave.

Detailed Walkthrough
Miner Extortion
When you first enter Karthwasten, you will see the townsfolk squaring off against a group of mercenaries sent by the Silver-Blood family in Markarth to secure the mine after a Forsworn incident. Ainethach, the owner of the mine, is arguing with Atar, the leader of the mercenary band:

Ainethach: "I want you sellswords out of my mine." 
Atar: ''"Watch your tongue, native. We'll leave when we're sure there's no Forsworn here." 
Ainethach: "Oh, and when would that be, I wonder? When I sell my land to the Silver-Bloods?" 
Atar: "The Silver-Bloods have made you a very generous offer for this pile of dirt. I suggest you take it." 

The Silver-Bloods are demanding that the mine be signed over to them in return for the protection they provided in the last attack. The mercenaries have been instructed to prevent anyone from returning to work in the Sauranach Mine until the rights to it are signed over to the Silver-Blood family. 

Deciding to Help Ainethach
Ainethach should be standing on the porch of his house during the day, and once approached he will agree to let you help him, and will ask you to convince Atar to leave. Unless you catch up to the band before they go into the mine, you will need to enter the mine to find Atar in the far end, standing on a raised platform.

Talking to Atar
After agreeing to help Ainethach enter the mine and speak with Atar. When you ask him to leave you will be given three options.


Note that even after successfully persuading, bribing, or intimidating, the mercenaries will claim that the mine is theirs, and will warn against sudden movements. There is no bounty for killing the mercenaries even after they agree to leave.

Strong Arm Tactics
An alternative option is to not talk to Atar at all, but to simply kill all the mercenaries. There are no mages or archers in the group, just weak melee fighters and the somewhat more formidable Atar, who is equipped as a bandit chief with leveled heavy armor, heavy shield, and one-handed weapon. The mercenary outside the mine will still greet you with pleasantries even after taking this option. There is no bounty for killing the mercenaries inside the mine, even if you attack first.

Selling Out
You can also remedy the situation by speaking to Atar who will offer you a cut of the purchase price if you can convince Ainethach to sell. You have to talk to Ainethach, and will be given the usual persuade, bribe, intimidate options if you tell him "I think you should sell the mine". 

After successfully convincing him, you will have to run back to Atar to tell him the good news, whereupon he will give you your cut.

Reward
The reward is the same regardless of which path you choose to take, a medium amount of leveled gold. Don't miss the seven silver ore veins in the mine and the leveled shield that Atar carries (if you decide to kill the mercenaries).
 
Helping Ainethach will additionally prompt Lash gra-Dushnikh into making you Blood-Kin with the Orcs (if you aren't already).

Notes
It is possible help both Atar and Ainethach and get both rewards of gold. Do this by first speaking to Ainethach and asking what the trouble is with the mine. Tell him you will convince Atar and his mercenaries to leave, which starts his quest, then go speak to Atar. Instead of telling Atar to leave, ask him why he shut down the mine and take on his quest to convince Ainethach to sell it. Go speak to Ainethach again and convince him to sell, then return to Atar and tell him Ainethach will no longer be a problem, completing his quest. As soon as you receive the leveled gold reward, kill Atar and his mercenaries, causing Ainethach's quest to reactivate. Speak to Ainethach again and tell him the mercenaries have been taken care of, which will grant you his gold reward as well. This method, however, does not always seem to work.
 

Quest Stages