Speak to Adril Arano
Notice! This mission can be completed only after March of the Dead and The Final Descent!
Enter Raven Rock and wait for Captain Veleth to approach you. He will tell you that Adril Arano - the Dunmer who greeted you on the island - is requesting your assistance. The worried man can be found walking around town, usually in one of the streets or at Morvayn Manor. Speak to him to learn of his dire situation - the hated Ulen family has returned to Solstheim. Your mission will be identifying the members of the family and serving them justice. Maybe Geldis Sadri is in possession of some useful information regarding them.
Speak to Geldis Sadri
In Raven Rock you can find a tavern called The Retching Netch. Geldis Sadri is the bartender there - and as every good one, he knows all the local gossip. Head to the lower level of the tavern and speak with the owner of this "cozy" place. The clever man will suggest that you should try sneaking to the Ulen Ancestral Tomb and wait for a member of the family to appear there.
If you ask Geldis about the tavern's specialty, you will receive an additional mission - Distribute Sadri's Sujamma. Take a look into it if you're running low on gold.
Wait inside Ulen Ancestral Tomb
Ulen Ancestral Tomb can be found in Raven Rock, right by the southern gate. If you head there during the day, you won't come across any obstacles. During night-time you will have to use lockpicks. After getting inside, wait for a mysterious character to enter the catacombs. No need for hiding afterwards - approach the woman and speak to her. Don't try killing her - she's immortal, so you will just waste arrows.
Report back to Adril Arano
After identifying the woman who visited the Ulen Ancestral Tomb, you can speak to her. Tilsu Severin - that's her name - is a very sly Elf. Whatever you say, she will turn it around and make you look like a fool. You won't be able to do anything (including killing her), so return to Adril Arano to give him a report. Find him on one of the street of Raven Rock or in Morvayn Mannor.
Tell Adril of what happened and you will receive another task - you need to head to Severin Manor and find proof that their members made contact with the Ulen family. The second councilor will give you a key, using which you will be able to enter the house of the supposed traitors.
Recover evidence from Severin Manor
Severin Manor can be found at the very edge of town - right by the descent to the Earth Stone. Head inside (using the key received from Adril) and you will be attacked by all the inmates. If you have Sneak developed high enough, you can silently kill or.. spare them all, stealing only the evidence without drawing anyone's attention. In the latter case, remember that you will be only putting the execution off in time - the same characters will be met during the later part of the mission and killing them is part of the story.
The evidence can be found in the sleeping rooms (the one with the manikins). Open the lock with a lockpick or using the key which is in possession of Mirri Severin (you can either kill or steal from her). Inside the safe, search for The Ulen Matter and take it. Afterwards return to Adril Arano and hand over the obtained evidence. Before doing that, you should take a look around the mansion - there are lots, literally lots of treasure to be found there.
Bring the evidence to Adril Arano
The title of this objective says it all - you have to deliver the obtained evidence (which you acquired in Severin Manor) to the quest giver. Adril Arano will quickly read the letter and tell you to "pay a visit" to the Ashfallow Citadel, where the enemies of the current councilor lives. In order for you not to feel lonely, a city guard will accompany you. Head to the point which Adril marked on your map and get ready for a rather hard fight with a large group of enemies.
Meet up with the Redoran Guard
Head out of Raven Rock and go east - Ashfallow Citadel, where you need to get, is found at the other edge of Solstheim. If you have already been there, you can use fast travel - otherwise you're in for a rather long trip across ashy plains. When you approach the main gate of the castle, be especially cautious - there are two Morag Tong assassins (a clan of assassins) lurking nearby. Killing them shouldn't cause you too much trouble - they have very little health points and light armors.
Approach the gate and you should find the corpse of a Redoran Guard on the stairs - it's the one who was supposed to accompany you. As that part of the plan went rather wrong, your only option is heading inside the castle and murdering everyone inside. However if you have some interest in mining (or blacksmithing), you should also take a look at the loads of heartstones which can be found beside the citadel.
End the threat to Councilor Morvayn
Head inside the stronghold and note the East Empire chest on a shelf on the left. Afterwards go down the round stairs and defeat the Morag Tong assassin who' standing there. Cross the tunnel which the enemy was protecting and sneak to the further part of the underground. There you will have to face two more assassins - you should defeat them easily. Search the room on both sides of the corridor and take form them everything that might come in handy (note that there's a safe in the room on the right). Afterwards pull the chain to open a passage to the next level of the catacombs (the switch can be found on the wall inside the room with the altar and alchemy table).
Here you will have to fight with three assassins - one is sitting on the gate, while the two others are training. Eliminate them and use two more switches. The first one can be found right beside the gate, while the second in the short corridor on the right (look out for snares!). Head to the next corridor and get ready for some gymnastics - there are tons of traps placed on the floor. Jump between them and enter the room on the right. Here you will also have to look out for pressure plates - avoid them and pull the two chains found on the stone pillars. That way you will create a passage to the big room at the end of the tunnel. Go there and get ready for the final fight with the enemies of Councilor Morvayn.
There are three enemies in the room - two Morag Tong assassins and Vendil Severin. Kill the last one is the objective in this mission. The man doesn't have too much health, but he's skilled at using two daggers and can cause you some problems. It's probably best to begin with taking care of one of the assassins (at best the one with the bow) and afterwards take care of the other opponents. If you're skilled at stealth, you should be able to do it with no problem. If you've chosen a different specialization, you can always go for a direct fight - drawing out one enemy at a time (they're less dangerous in doors). Keep in mind that the enemies are as vulnerable to traps as you are - luring them into the spiked corridor is a great idea.
After taking care of them, search their bodies and the chest and return to the quest giver. The mission is nearing its end.
Return to Adril Arano
After taking care of business at the Ashfallow Citadel, you can return to Raven Rock. Adril Arano at one of his favorite places and report back to him. The man will be sad because of the death of the Redoran Guard, but you can cheer him up with the news of Vendil Severin's death. Now for the hard-earned reward.
Follow Adril Arano
Completing this part of the mission will take you just a couple seconds. Follow Adril Arano to Morvayn Manor and listen to his conversation with the Councilor (you can notice that they seem to be good friends). In the end the Councilor will speak to you directly - listen to him closely!
Speak to Councilor Morvayn
Speak to Councilor Morvayn to end the mission and receive an appropriate payment (depending on your level). What's more, in return for your deeds you will be rewarded with the Severin Mansion (which you should know fairly well by now). Inside you will find a smithing suite, alchemy lab, arcane enchanter and many, many more things. Apparently you've become one of the most important people in Solstheim!