- Meet Mercer Frey outside Snow Veil Sanctum.
- Enter the Sanctum with Mercer.
- Find Karliah.
- Speak to Karliah.
There’s a note that you should speak with Tonilia about upgrading your armor. Head to the Flagon and find her about the room. You get one choice about which piece of the Thieves Guild armor set to upgrade:
- The upgraded guild armor increases your carrying capacity from 20 to 35
- The upgraded boots increase Pickpocket from 15% better to 25% better
- The upgraded gloves increase Lock picking from 15% easier to 25% easier
- The upgraded cap alters prices from 10% better to 15% better
Journey to Snow Veil Sanctum, which is northeast of Windhelm (on the same side of the river). The easiest walking path is to start from Hlaalu Farm and look across the river to see the start of the path heading up the mountain. You may encounter Horkers, Snow Wraiths, and Saber Cats. Near the Sanctum, you will find an abandoned camp and a dead horse, which Mercer will later tell you was Karliah’s.
Entering the Sanctum requires Mercer to open the lock, which otherwise says a key is required. This may require some patience, as Mercer has to come down the stairs and interact with the door. Engage Mercer in conversation to learn more about Karliah.
Proceed inside with Mercer (who will replace any other companion you may be traveling with). You will face the usual Draugrs and Restless Draugrs, but will also encounter Draugr Wights and a Draugr Overlord, as well as tripwire, bear traps, and one trap that drops a flammable substance. Mercer will make snide remarks about your sneaking abilities.
When Mercer tell you to pull the chain at the first gated door. Before you do so, note the iron spike wall behind the pots to the right of the pull chain. This bar will swing around and smash you into the wall doing a decent dent in your health if you don’t make sure to be out of its way when activating.
The chain to the second metal gate is easy to miss, especially since Clairvoyance will point you into a right-side wall. The chain is actually to the left of the metal gate in an alcove. Up ahead you will have the option of proceeding through a metal door to room with treasure and a perch to take out quite a few of the Draugr below before proceeding back down and into the large room below. At higher levels, there is a Dragon Priest that spawns by the word wall, just be prepared for one.
You will encounter a puzzle-ring lock, similar to the one faced in The Golden Claw, but it is not necessary to decipher it; Mercer will open it.
Finally, you will come to a set of metal doors. As you step through, Karliah shoots you with a poisoned arrow; the scene fades to black. In a cutscene, you lay senseless but are witness to a conversation between Karliah and Mercer in which you learn that Mercer is the one who killed Gallus (the former guildmaster). Karliah turns invisible and walks away; Mercer tells you that the others will hear how you died a hero, then slits your throat and leaves you to die.
You regain consciousness just outside the entrance where you began with Karliah standing over you. She explains that she concocted a unique paralysis poison that not only rendered you motionless, but slowed your heartbeat enough that you did not bleed out from Mercer’s coup-de-grace. (She’d intended to shoot Mercer, but he was careful to use you as cover.) She explains that Mercer killed Gallus over an argument about methods, and that Gallus left other secrets in his journal, which is in code.
Karliah sends you to Winterhold, where a mage called Enthir who used to be friends with Gallus Desidenius (former guild master) may be able to help.