Scoundrel's Folly

Walkthrough
The development so far still did not reveal the name behind the mysterious marking on the Promissory Note, the same marking already seen on Goldenglow Bill of Sale. The latter did bring up the name Gulum-Ei though... a possible lead.

Speak to Mercer Frey
Mercer Frey recognizes the name Gulum-Ei as that of a former Guild informant who kept an eye on the East Empire Company activities, passing along some of their valuables. But recently his contributions to the Guild have been dwindling. Mercer suggests a stern talking to Gulum-Ei by the Dragonborn.

(Optional) Speak to Brynjolf
Talking to Brynjolf reveals a few more details on Gulum-Ei and the East Empire Company Warehouse in Solitude.

Speak to Gulum-Ei
Gulum-Ei can be found at The Winking Skeever in Solitude. At first he will faint complete ignorance. The mention of his name on the Goldenglow Bill of Sale slightly refreshes his mind, but he does not give the matter much thought.

To get more information out of Gulum-Ei the Dragonborn can persuade, bribe or intimidate.

Steal case of Firebrand Wine
Depending on the Dragonborn's prior encounters with people at the Blue Palace, this may not be considered theft.

Get information from Gulum-Ei
After a successful persuade Gulum-Ei mentions that a woman approached him with a lot of money, asking him to act as broker in the matter, and to pay Aringoth for the estate. He did as he was told, did not ask any questions and returned with the deed. The woman was furious at Mercer Frey though. As that may be Gulum-Ei never found out the name of the woman... or so he says.

Shadow Gulum-Ei
The Dragonborn is not convinced that Gulum-Ei is telling all he knows, and thus decides to tail the Argonian. Gulum-Ei leaves the inn, calmly walking to the East Empire Company Warehouse, that is presently not guarded at the main gate.

Tip: Following Gulum-Ei involves sneaking, or at least not getting noticed. In the warehouse the Dragonborn needs to get past the guards without being noticed, to not alert Gulum-Ei. So telling any follower to stand and wait outside the main warehouse entrance it probably a good idea.

Tip: On the north-eastern side of the warehouse cave, up some wooden steps there is a wooden house that contains the East Empire Shipping Map, an unusual item Delvin Mallory will be delighted to purchase. (A revisit might best be done towards the end of the quest, before returning to Mercer, though.)

Following Gulum-Ei through the warehouse, avoiding the guards, he finally exits the cave via a difficult to spot path behind some shelves, to enter the Brinewater Grotto. After getting past ten bandits, sneaking might well be an option, find Gulum-Ei at the other end of the grotto.

Confront Gulum-Ei
When the Dragonborn approaches Gulum-Ei, the Argonian will quickly cave in. One can choose to placate the fearful Gulum-Ei by promising to not mention anything incriminating to Mercer, neutrally request information first and to then decide what will happen with him or outright threaten him.

As it turns out the woman pulling all the stings in the background is called Karliah, and apparently she killed the last Guild Master before fleeing.

Should the Dragonborn have chosen the mild approach with Gulum-Ei, he will offer his services as a fence in the future.

Find alternative source of information
Killing Gulum-Ei requires a different approach... ???

Return to Mercer Frey
The Dragonborn returns to Mercer Frey with the name of the mysterious adversary who seems to want to bring down the Thieves' Guild. Mercer is quite shocked to hear the name Karliah, a name he had hoped never to hear again, and after some prodding tells the story of how both of them used to be partners in the Guild until she killed the Guild Master, Gallus Desidenius. Her motives seem to be revenge, though Mercer can only guess at them.

Anyway, as reward for the information Mercer instructs Tonilia to upgrade one piece of the Dragonborn's Thieves' Guild Armor Set, i.e. either the gloves, hood, armour or boots.

Mercer decides to take out Karliah to end the conflict once and for all, and the Dragonborn has the honour of coming along in Speaking With Silence.

Ein einfältiger Halunke