A Cornered Rat

Description
During Diplomatic Immunity, the Dragonborn will find out about Esbern, an old friend of Delphine and the only other member of the Blades in Skyrim. Delphine will tell the Dragonborn to go to the Ratway in Riften to find him before the Thalmor do. Thalmor Agents will pop up outside as well, and once the Dragonborn has talked to Esbern, the agents will attack.

Locating Esbern
Esbern is located in The Ratway Warrens behind a barred door with a viewing slot. Access the Ratway Warrens by traveling through several areas, including the Ratway, The Ragged Flagon, and The Ratway Vaults. The Ratway's main entrance is located on the south end and lower level of Riften.

Brynjolf, if not spoken to already, will be located in town during the day and at The Bee and Barb at night. Talking to Brynjolf at the Bee and Barb will trigger conversation leading to the Thieves Guild quest line. If the Dragonborn has already become a member of the Thieves Guild, they can talk to Brynjolf or Vekel the Man inside The Ragged Flagon - Cistern to advance to the next part of the quest.

Alternatively, chatting to Keerava, the barmaid of the Bee and Barb, can give the Dragonborn the information without putting the question to Brynjolf, although this will bypass the introduction to the Thieves Guild. Once inside the Ragged Flagon, talk to Vekel about Esbern. He will not give information about Esbern unless the Dragonborn has a high Persuasion, 140 or beats him in a Brawl. Then, he will give Esbern's location and warn of Thalmor looking for him as well.

After meeting with Brynjolf and mentioning Esbern, he will give information that Esbern is in the Ratways, and the quest marker will update accordingly. There have been problems reported with quest advancement if the quest A Chance Arrangement has also been picked up. A Cornered Rat can progress after completing A Chance Arrangement or after being caught. Brynjolf then will ask the Dragonborn if they want to join the Thieves Guild (if this hasn't been done already), and a new quest will start. The slot on Esbern's door will be closed, and conversation with Esbern is not possible if the quest has not advanced properly.

Ratway
Some events in the Ratway may unfold differently depending on certain events during Diplomatic Immunity.

If this is the first time inside the Ratway, there will be a few enemies to fight through to get to the headquarters of the Thieves Guild. If already a member of the Thieves Guild, the Dragonborn may enter through the secret entrance in the Riften graveyard.

If the Dragonborn did not kill a man named Gissur during Diplomatic Immunity, It is possible to be attacked by him outside the entrance to The Ragged Flagon. Inside, after the attack, Gissur may be identified by Guild members and killed. The Dragonborn can take Gissur's Note from his body. The note may also be found in the Ratway without the body. If Gissur was killed at the Thalmor Embassy, he will not appear during this quest.

From the Ragged Flagon, head through the Ratway Tunnels to get to the Ratway Warrens. It may be necessary to fight several Thalmor while navigating through the tunnels before locating the entrance.

Once in the Warrens, locate a locked door with a viewing slot at the top of a set of steps and activate the door to engage Esbern in conversation. The Dragonborn will then be able to tell Esbern about Delphine and warn him of the Thalmor searching for him. After being convinced, there is an unusually long delay as he gathers his thoughts and unlocks what seems like an absurd number of door locks and bars.

Once inside, the conversation will continue as he gathers his belongings. He will be able to provide some additional background information about the Blades, and the Dragonborn will bring him up to date on current events. Once the conversation with Esbern has ended, this quest will be completed and Alduin's Wall will begin.

Bugs
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 * The conversations with Esbern may fail to continue or complete, with Esbern freezing.
 * Esbern will search his treasure, desk, and lab area of his room - if his conversation cannot be heard there is no way to know that he's actually talking while he does this - it may seem like he is frozen. The same is true after bringing him back to Delphine; he will be talking to her but the conversation cannot be heard.

In some cases bugs can be avoided altogether by using console command, which forces Esbern to skip all dialog, open the door, and follow the Dragonborn to Riverwood.

Esbern's door

 * In first conversation, Esbern is voiceless and after the conversation he fails to open the door. It may take several minutes before the door opens as scripted.
 * If waiting doesn't work try using the TCL command to walk through the door. Shut down the TCL, talk to him, when he starts using the chest, kill him. Then talk to him again and tell him to lead the way.
 * Instead of using TCL, you can disable and re-enable the door.
 * This can be sometimes fixed by deleting (BACKUP FIRST!) "Skyrim.ini" and "SkyrimPrefs.ini" from "C:\Users\[USERNAME]\Documents\My Games\Skyrim". All preferences are now default. If everything works it is recommended to set up preferences again rather than overwrite new files with backup.
 * If the door does not open, face door type  force Esbern out of the door and close it. He will then open the door to get back in, he then will continue on with the quest though he may speak too fast to understand.

Esbern's voice

 * It is possible to correct speech issues with Esbern by installing a 3rd party speech mod.
 * There is another fix to this problem: using a game manager (like the Fallout Mod Manager). Extract the "voices extra" file in the "data" folder of the game's program files into the same folder (doing so places a folder named "sound" in it) and it will be fixed completely.
 * Esbern may keep saying "come in" constantly.

Esbern frozen or stuck
Each time he seems to be frozen, try one of several reported fixes:
 * Make sure everything has been killed in the Ratway beforehand; otherwise, Esbern instantly summons a Frost Atronach in combat which will get him stuck occasionally.
 * If he gets stuck just start a dialogue with him, keep doing it if it continues.
 * If Paralyze is cast on him he will not turn hostile, and upon regaining his muscle control, he will skip to his next task speeding everything up.
 * If he ever gets stuck kill him. If killing doesn't work try failing to pickpocket/place an item.
 * Try to push him by using sprint or Unrelenting Force, to make him walk again if he stops.
 * It is possible to first make Esbern follow by opening the console, clicking on Esbern, typing disable, then typing enable. He will then proceed to his next "task". This may have to be done multiple times before he will follow properly.

Quest issues

 * Sometimes this quest will fail to start at all and never even show up in the quest log after completing Diplomatic Immunity, in which case it will skip ahead to the next quest, Alduin's Wall. In the dev console, type.
 * After speaking to Delphine about Esbern, she will direct the Dragonborn to Brynjolf, but there will be no mission start and it will not be possible to ask the bartender in the Ragged Flagon or The Bee and Barb about Esbern. This may be a result of having Darkness Returns active at the time.
 * Esbern may say "go away" anytime his door is activated if the quest hasn't been advanced enough or it is not currently active.
 * If the Dragonborn goes straight to the Ragged Flagon after escaping the embassy, then the quest Diplomatic Immunity may end without Delphine telling him/her to mention 'the 30th of Frostfall'. If this occurs, Esbern will refuse to acknowledge the Dragonborn and the quest will become stuck.

General

 * If a follower gets into a fight with him, switch the scene and come back, Esbern will appear as normal again.
 * Sometimes Esbern may conjure/summon a friendly Storm Atronach. If the Atronach gets caught in one of Dragonborns shouts or is accidentally hit by the Dragonborn, it will turn hostile and attack friendlies.
 * Very rarely during this quest, when exiting the Ratway, the player will get stuck half below the passage outside. The visuals are as if the player is no-clipping, as you can see the bottom of the city, and you cannot walk out; you are stuck. The easiest way to stop this is to load to a previous save and try again.
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Крыса, загнанная в угол Eine Ratte in der Klemme Una rata arrinconada