Summerset Shadows

Walkthrough
After getting this special contract from Delvin Mallory, after completing 5 Thieves' Guild quests in Windhelm

Speak to Torsten Cruel-Sea
You will need to speak to Torsten Cruel-Sea in Windhelm or the farms just south-east of the city. He tells you that his daughter, Fjotli, has been slain and robbed of a silver locket, a family heirloom, by a thieves' guild hailing from the Summerset Isles. These thieves call themselves "The Sumerset Shadows", and are looking at expanding their influence in Skyrim.

Speak to Niranye
He then suggests that you to talk to Niranye at the marketplace who also originated from the Summerset Isles. Apparently she was forced into helping this guild, but needs to be persuaded or intimidated to provide the location of their base of operations.

After the quest Niranye will become a fence for the Thieves' Guild.

(Optional) Discover leverage to sway Niranye
???

Travel to Uttering Hills Cave
It turns out they have holed up in the Uttering Hills Cave.

Recover Fjolti's Silver Locket
Enter the cave and make it past many of the members. Early on in the structure you may directly stumble into the guild leader Linwe. Take him out and recover the locket.

Note: Linwe has his own named armour set... Linwe's Armor Set.

(Optional) Destroy the Summerset Shadow's Banner
Once you get to the room with bedrooms either side you will find a banner, with an optional objective to destroy it. Use a flame destructive spell on it.

Return to Torsten Cruel-Sea
Return to Torsten Cruel-Sea, and give him Fjotli's Silver Locket. Though still sad he will promise to support the Thieves' Guild in Windhelm, and rewards the Dragonborn with the Ring of Deft Hands.

Consequences
Delvin Mallory will be pleased with the outcome, assuring the Guild's foothold in Windhelm. Though he can point out that the Dragonborn may not have cleaned up with the other guild thoroughly enough (not killing all members and/or not destroying their banner).

He will also mention that the fletcher Syndus has taken up residence in the south-west cell, just outside of the Ragged Flagon.

Schatten von Summerset