Speaking With Silence

It is offered by Mercer Frey immediately upon completion of Scoundrel's Folly quest.

Objectives

 * Meet Mercer Frey outside Snow Veil Sanctum
 * Enter Snow Veil Sanctum
 * Find Karliah
 * Speak to Karliah

Walkthrough

 * Have at least one piece of the Thieves Guild armor you were given. You will need to trade a piece with Tonilia for something better.
 * Meet Mercer Frey outside Snow Veil Sanctum. Talk to him, and he will unlock the door. Enter it and fight through the Draugr with him. There will be a Larceny Target on a trapped pressure plate, specifically the Model Ship which you can pick up and sell to Delvin for a leveled amount of gold (300-800). At some point you will encounter a locked puzzle door, which usually requires a dragon claw made out of a specific material. However, this time Mercer will open it. Upon advancing after the locked door, you will be shot by a poisoned arrow and fall to the floor, you will be able to watch Mercer's and Karliah's meeting and learn more about his real intentions. After their talk Mercer will approach you and stab you, the screen will then black out. You will wake up outside the Sanctum in a small camp, having been rescued by Karliah. Speak to her, and find out about Mercer Frey's treachery to the guild.
 * Note: Your followers will wait outside while you are in the dungeon with Mercer Frey.
 * Note: If Mercer Frey does not open the door and you have a follower, ask the follower to "Wait here.", otherwise dismiss the follower (do the same if a dog is following), and Frey will open the door, otherwise he will follow for the rest of the game.
 * Note: If Mercer Frey does not move, he can be pushed into Snow Veil Sanctum with unrelenting force, then into the door by walking into him.
 * You cannot pickpocket Mercer Frey until after he opens the final puzzle door, and after that you can pickpocket his Dwarven Sword of Devouring (Damage: 13, Weight: 12, Value: 874, Absorb 15 points of health) and his iron dagger. If you do, the dialog and animation still follow, but he threatens Karliah with his fists and cuts you with an imaginary blade.
 * After Mercer Frey unlocks the puzzle door, you can take him back to the previous pull-chain-activated gate and lock him behind. Karliah will still shoot you with the poisoned arrow. The dialog will follow but Mercer Frey will not be present.

Trivia

 * Snow Veil Sanctum contains a Dragon Word Wall. (Disarm). In one of the rooms in the Snow Veil Sanctum, there is a raised platform with some Draugr. Behind that there is the word wall.
 * Snow Veil Sanctum has a few tripwire-based traps. Keep an eye out!
 * There is a Larceny Target in Snow Veil Sanctum, more specifically the Model Ship. Be wary, as the explosive oil trap in that room can send the ship flying.
 * Snow Veil Sanctum is on the Dungeon Lock Outs list.
 * Once end scenes are over there should be a dead horse outside behind you, when you're by Karliah, and the horse should have some coins and possibly potion(s).
 * The room in which the player is shot by Karliah contains only a skeleton with minor random loot.
 * In the same room as the word wall there is a skill book for light armor.
 * If you tried to Whirlwind Sprint where Karliah shoots you, the game lags for a moment. Everything still works normally.
 * It is possible to get Mercer's sword early. In the Word Wall room, 3 Draugr will wake in this room. At least one of these Draugr can, and will, use the Disarm Shout. You can lure him to Mercer, and if the Draugr does use it on Mercer, his sword will go flying, allowing you to find and retrieve it. This causes a rather peculiar change later in the quest, where Mercer stabs you.

Bugs

 * Sometimes Mercer Frey does not immediately unlock the door to Snow Veil Sanctum. This can sometimes be fixed by saving and reloading, although that is not always the case.
 * Console command for unlock can fix this.
 * Upon reaching the cutscene with Karliah, it is possible that the cutscene will freeze.
 * Waiting for a number of hours may fix this bug.
 * Karliah may become stuck inside Snow Veil Sanctum.
 * After the cutscene where Mercer stabs you, the camera will be zoomed to first person and you won't be able to zoom back out.
 * A solution may be saving and reloading.
 * Another solution to this bug is by turning into a werewolf: when the effect wears off, you will be able to go to 3rd and 1st person as usual.
 * Console Command "tc" freecam would also fix this bug.
 * The floor can be found to be missing random chunks, causing Mercer to fall in.