When you next speak to Mercer, he will remark that the symbol found on the Goldenglow Bill of Sale from Goldenglow Estate is the same as the one on the Promissory Note found at the Honningbrew Meadery.
This leads him to the conclusion that the Thieves Guild is not, in fact, cursed, as Delvin believes, but is instead being deliberately acted against by an unknown competitor. Mercer will also reveal that a name mentioned on the Bill is the alias of an old corroborator of the Guild; an Argonian dockworker for the East Empire Company by the name of Gulum-Ei. He wants you to go and speak to this individual.
Gulum-Ei can be found in a small alcove within Solitude’s inn, The Winking Skeever. If your Speech level is high enough, you can easily get the needed information from him using the [Persuade] dialogue option ([Intimidate] won’t work). Otherwise, you’ll have to steal a case of Fire Wine from Blue Castle in Solitude. This is relatively easy, as the case is sitting out in the open upon a small shelf in the left-hand hallway off of the main room. There are guards patrolling the area, so it’s best to be sneaking when you snatch it.
Once Gulum-Ei gets up from his table and makes his way to the door, you are given the objective to shadow him. He ends up leading you to the East Empire Company Warehouse, where you must then pick the door to enter the building.
If you’re following Gulum-Ei closely enough, you should still be able to keep your eyes on him as he walks inside the warehouse (but don’t follow him from just a few feet behind so that he’ll see you if he turns around, do the thing properly). The guards in this location are hostile, so if they spot you, kill them immediately. Continue following Gulum-Ei until he exits to Brinewater Grotto. If you lose sight of him, take note of the stairs leading to the water and the ramp across it with a draft blowing.
Upon entering, the game notifies you that you have completed the objective of shadowing Gulum and are given a new objective: confronting him. You cannot turn back at this point, as the door has now locked behind you and requires a key to unlock.
Making your way through the Grotto, you will come across multiple pairs of Bandits and Highwaymen whom you are free to kill as you wish without consequence to the quest. Near the end you will come across a grotto with some cages and shelves containing a few chests, some armor and other items that you are free to loot. Gulum is standing there with a bandit. Attack the bandit and Gulum will flee from the fight. After you have dispatched the bandit, pursue and speak to Gulum. You can choose to kill him, but doing so will get you nothing, whereas if you let him live, from now on he will act as a fence for you.
After you have finished with Gulum, get out of the room by pulling the middle one of three levers nearby to expose a hole in the wall that leads to an exit. The two levers on either side of it just open the empty cells. Head back to Riften and speak with Mercer to complete this quest.
Your reward for the quest is an upgraded piece of Thieves Guild armor. To receive it you must speak to Tonilia and trade in one of your current pieces for an upgraded version. You are able to choose which piece you want to upgrade.