The Pursuit

It is offered by Karliah upon completion of the previous quest, "Hard Answers".

Objectives

 * Meet Karliah at the Ragged Flagon
 * Follow Karliah
 * Speak to Brynjolf
 * Infiltrate Mercer's House
 * (Optional) Speak to Vex about Vald
 * (Optional) Shoot the mechanism to lower the ramp
 * Discover evidence of Mercer's location
 * Speak to Brynjolf

Walkthrough

 * 1) This quest begins with the first objective already in play. When you return to The Thieves Guild where you'll find your usual entrance point by the Hall of the Dead Mausoleum is locked. You must go through the Ratway, down to the Ragged Flagon, where you'll meet Karliah. She has Gallus's translated journal ready to show the Guild members who require convincing. Agree to back her up and move into the Cistern. As expected, Brynjolf, Vex, and Delvin Mallory are waiting for you with their weapons drawn. Karliah hands over Gallus's Journal as proof that it was Mercer that had been stealing from the guild.
 * 2) Brynjolf cannot believe Mercer has been stealing from the Guild, so he orders Delvin to open the vault, which is found empty. It takes two keys to unlock the vault. Delvin, Brynjolf, and Mercer are the only ones who carry such keys, so how did Mercer break into the vault alone? The answer is a mystery (even though Karliah quietly has her suspicions). Although vindicated, Karliah is just as angered by this as the rest of the Guild, except perhaps for the seething Vex, who vows to kill Mercer immediately. Brynjolf' tries to calm the guild down, and he orders Vex and Delvin to guard the Ragged Flagon. Then he turns to you and asks what you've learned from Karliah. Explain that Mercer killed Gallus, that the three of them were Nightingales, and that Karliah was behind Goldenglow and Honningbrew, which was a cunning plan to try and make Mercer look weak in front of Maven Black-Briar. Then Brynjolf has an important task for you: Break into Mercer's Riften house - Riftweald Manor - and gather any information that may indicate where the traitor has gone. Before you leave Brynjolf, ask him what is the best way into Riftweald Manor. You receive information about a "watchdog" and an exterior ramp with a mechanism to lower it, which could aid in your escape. Then ask about the watchdog named Vald. Brynjolf indicates Vex may have more information to provide. These grant you two optional objectives.
 * 3) Locate Vex in the Cistern or Ragged Flagon, and ask her about Vald. She tells you he's only interested in gold, so buying him off is a possibility, but it won't be cheap and Vald might not betray Mercer. So Vex suggests you speak to Maven Black-Briar about erasing Vald's debt, or run him through with a blade and take what you need off his body.
 * 4) Work your way around Riften until you are behind Riftweald Manor. There are many entrances into the area. You go through the expert level lock into the back, or there is a gate that requires a key. If you use a bow and arrow to shoot down the ramp Vald will come through the gate to look for who did it. Hide from him and sneak into the manor, you can either pickpocket the key from Vald or lockpick the door which has an expert level lock.
 * 5) Once inside the manor you can hear voices right away, beyond the next door is a guard that is looking in the opposite direction. Sneak around to the bedroom or behind him and kill him using a sneak attack, or try to sneak by him to the next room. Go down the stairs and ignore the guard at the other end of the area, go through a closed set of doors. On the far left of the room there will be a suspicious cabinet, open it and push the back out to find another set of stairs to go down. You will see a large opening into a series of tunnels. It isn't hard to navigate through, but watch your step for a tripwire that will activate a series of blades. Soon you will reach a doorway, stand right next to the doorway as it is rigged with arrows that shoot out once it is opened. There is a lot of valuable loot in the room, as well as a Larceny Target the Bust of the Gray Fox which should be taken to Delvin for some leveled value.
 * 6) Continue onto through the last tunnel and you will be at the door to The Rataway Vaults. Follow the quest objective marker back to Brynjolf talk to him and the quest will be completed.

Bugs

 * Sometimes, after talking to Karliah and Enthir at the end of Hard Answers, Karliah will not leave The Frozen Hearth's basement, making the quest impossible - she will not initiate the correct dialogue, or move to the Ragged Flagon. An easy way to tell if this glitch has happened is to check the map - if the pointer to talk to Karliah is at The Frozen Hearth, she's still there. One way to fix this on PC is putting these commands through the console: setstage TG07 20, setstage TG07 30. After this, find Brynjolf, talk to him, and the quest will proceed as normal.
 * I found out that this glitch aslo affects the Dark Brotherhood's quest when you got to decorate the Dawnstar Sanctuary. Delven wont talk tou you till he "figures things out"
 * There is a way for X-box and PS3 users to fix this glich. Go back to the place where you did the charcol rubbing of the Falmer language thing ( in Markarth i think). Do another rubbing, it will reopen Hard Answers. You then redo the quest to talk to Karlia and Ether and make sure that she leaves, then just travel to Riften and wait. (Tested on PS3) PS. I read this on a a Forum, I have not personaly tested this. Try it for yourself to verify.
 * Sometimes the entrance to the Thieves Guild by the Hall of the Dead will not be locked.
 * There exists a similar bug when you tell Karliah to wait before confronting the guild. The quest progresses fine up to the point where Karliah is waiting in the main area of the Ragged Flagon, but if you put this quest off for several levels and tell her to wait there until you're ready, she may go back to the Frozen Hearth Cellar. This can be fixed by telling her you are ready (while she is in the basement of the inn), then entering the Ragged Flagon Cistern. Delvin will walk up and start his dialogue as if Karliah is there. This will trigger Karliah to move from the inn, and she will (slowly) walk to Riften and enter the Cistern after several in-game hours. Quest will resume normally after this.
 * If you try to shoot the gear and miss, Vald might run up to where you are. He will open the gate and attack you. The guards also will kill him quickly with no bounty on your head. (Note that I didn't attack Vald.) allowing for easy entrance.
 * Sometimes Karliah doesn't give you the quest if you already have "Mercer's Plans" in your inventory. Fix this by dropping the plans before completing Hard Answers.
 * Looking around for Mercer's manor, not knowing it was the building right next to the cemetery, Vlad randomly targeted me and the guards told me "you should probably get out of here" and when I turned around and attacked Vlad I got a 1-hit kill, I did not try breacking into the manor or eve shooting at the gear.