Where to start the quest: The Restful Watchman
Person giving the quest: Dumrag Gro-Morrg
In the tavern you can meet an orc mercenary that will tell you about how he was attacked by a group of other mercenaries. His wife is now imprisoned and he barely survived the battle. He asks you to help him find his wife - go to the castle and ask nobles if they know something about this incident.
When you are inside of the castle you should go to the Banquet Hall (next to the Throne Room) and speak with Affana Eddici. The woman tells you that one of the nobles recruits mercenaries. She recommends you to read his journal and she gives you a key to his room. In exchange she asks you to leave her note there. You should agree, thus will make things easier for you.
You should now head to Guest's Wing. Approach the closed door and use the key. Next, you must go to the desk and take Simund's journal. You should also leave Affana's note there. After that you must return to the main hall and speak with Rignar. Convince him that you work for a good cause and he will go to check his registries. Wait for him and when the man returns he will confirm your suspicions regarding Simund.
You must now go to Dumrag and tell him about everything that you have learned. The orc won't believe you that Simund is responsible for the ambush. He tells you to go to his mansion and gather potential evidence. The mansion is located in mountains, east from Bruma. When you approach the mansion you will be attacked by a guard. After the fight you must enter inside the building and defeat another guard. Then, you must go upstairs. Take a letter from a desk which confirms Simund's fault.
Return to Dumrag and the orc will get angry. He wants you to go with him to Simund's house and get revenge on him. Go back to the mansion with the orc. Dumrag, during his search, finds a trap door. Destroy the barrel and go down.
The discovery, that you make, is really something - Simund and Dumrag's wife are having an affair. After a fuss the orc will ask you what he should do. You can tell him to forget about his revenge, order him to kill Simund or say that he should make a choice himself (in this case he kills both characters). After making a choice you receive a reward from Dumrag and the quest will end.