Diplomatic Immunity

Quest progression
Objectives:
 * Meet Malborn in Solitude.
 * Give Malborn your equipment.
 * Meet Delphine at Katla's Farm in Solitude.
 * Create a distraction and get away from the party.
 * (Optional) Retrieve your gear.
 * Search for information about the Dragons returning.
 * Escape the Thalmor Embassy.
 * Talk to Delphine.

Background
"It seems that things are worse than even Delphine thought. Dragons aren't just coming back to life, they're being resurrected by other dragons. Delphine suspects that the Thalmor might be bringing dragons back for some sinister purpose. She's going to help me infiltrate the Thalmor ambassador's manor and find out what they know about the dragons. I should meet with her contact, Malborn, in the Winking Skeever Inn in Solitude."

Malborn
Delphine thinks that the Thalmor know something about the Dragons' return to Skyrim. She has a plan to get you into the Thalmor Embassy, so that you can get hold of secret documents to find out more. She will acquire a legitimate invitation to a reception being held by the Thalmor ambassador, Elenwen. Find the elf Malborn at the Winking Skeever tavern in Solitude. When you arrive Malborn will agree to smuggle some of your equipment into the party. Malborn can bring a significant amount of your gear with him, but because of all the loot you can grab during this mission (including sets of Elven weapons/armor, Fine Clothes/boots/hats etc.), you might want to give him only the bare necessities in order to save space. Although if you are quite wealthy already, the value of the obtainable loot is of considerably of less importance and you need not overly concern yourself with this factor. However if you are not so wealthy, you may want to consider bringing the minimum of armor, weapons, lockpicks, and whatever scrolls or potions your skill level requires; the only gear you'll have during the mission will be what you give Malborn. Afterwards, you'll meet with Delphine at the Solitude stables at Katla's Farm to pick up your invitation and party disguise, and drop off everything else in your inventory. Extra gear can be left at your residence before meeting Delphine at the stables, or automatically given to Delphine for safekeeping at her safehouse; everything you give her will be stored safely until you claim it after the mission. It should be noted that Delphine will store the inventory in a chest in her office at the Sleeping Giant Inn, in Riverwood.
 * Quest items, such as the Skeleton Key, cannot be removed from your inventory—useful if you forgot to bring lockpicks.

Once inside the party, you will be approached by Ambassador Elenwen and engaged in a conversation. Malborn interrupts the conversation, causing the ambassador to walk off. You then have the opportunity to speak with Malborn and ask him for a drink — it could come in handy.

Distractions
Ambassador Elenwen greets you upon entering the Thalmor Embassy. After speaking with her, you'll need to create an opening to slip out of the party; tell Malborn when you are ready, and he'll wait by the door behind the bar for your move:
 * If you talk to Razelan on the bench nearest to Malborn, he'll ask for a drink. Malborn can conveniently provide you with a Colovian Brandy to give to Razelan, who then offers to do you a favor in return. Getting a drink from the Wood Elf servant and giving it to Razelan will also work. Tell him to cause a scene, and he will make a grandiose announcement, giving you your distraction.
 * Alternatively, if you are acquainted with Jarl Idgrod Ravencrone and have previously assisted her, you may ask her to cause a distraction, as "an old woman can get away with anything". She will then begin harassing Razelan and making a scene, providing you the necessary cover.
 * Another option is to overhear Erikur trying to flirt with the Wood Elf servant Brelas, unsuccessfully. Speaking with Erikur afterward will make him mention his weakness for Bosmer women, and you can therefore offer to speak with her on his behalf. Speaking to Brelas reveals her dislike for guests like Erikur and asks you to persuade him to leave her alone. Speak to Erikur once again, and any speech option chosen will cause Erikur to lose his temper and causes a scene.
 * In addition, if you are on good terms with Ondolemar, you may ask him to make a distraction (after a persuasion check) where he begins accusing Razelan of insulting the Thalmor.

Once you have your opening, run to the door behind the bar and follow Malborn. Proceed through the kitchen to a pantry, retrieve your smuggled equipment from a chest, and go through a door into the next hallway. If you did not give him any of your belongings, the rest of this quest will be quite difficult. Remember that you still have your magic, including shouts, and Beast Form (if you have it) will be indispensable if you have no armor and weapons. Malborn will lock the door behind you, completing this phase of the mission.

Thalmor espionage
Having Aura Whisper or Detect Life can be handy for this phase, but not necessary, as there are few guards in any given area. Thalmor Guards are pretty hard to kill at low levels, but their armor sells a lot and is worth every kill.

As Malborn locks the door behind you, you will see a doorway to your left and overhear two Thalmor agents chatting over a bar. Behind the bar are stairs leading up to a Thalmor Wizard. Kill them (or wait for them to move out of the way and sneak past them), and then either go through the ground floor exit or turn left at the top of the stairs, kill the mage (as he never leaves the narrow hallway and is hard to not brush up against even if you have access to invisibility), and then turn left again to enter a room with access to a second floor exit. This will take you to an exterior area with more enemies to kill or avoid. The two guards can be avoided, but a mage guards the door and will alert the guards if you attack him, so it's probably easier to take out the guards first or distract the mage with an arrow or the Throw Voice shout if you have it. If a general alarm is raised, additional guards will come from the barracks. Once clear, enter the only door into the Ambassador's offices (Elenwen's Solar). If you are lucky, a random dragon may spawn in the area outside and start attacking. This can create an excellent distraction to slip into Elenwen's Solar as all the guards, including the mage guarding the office door, will become busy fighting it. However the possibility of this event happening is rare. Alternatively, if you are an Altmer and are wearing Hooded Thalmor Robes, you can simply walk past all the guards without incident. Still as an Altmer, once you reach the door to Elenwyn's Solar the Thalmor Wizard will ask you why you are not at your post at which point you have the option to persuade that Elenwyn wants to see him inside. If this persuasion is successful the guard will ask you to "take his post" and tell you not to let anyone in at which point he should leave the area but appears to stay where he is. Either way, once you enter the Solar, your disguise will no longer work and everyone inside will be hostile to you.

Inside, you may kill everyone or sneak over to the chest the quest arrow is pointing to and grab the items you need. Read the documents taken to activate the next arrow. If you choose not to kill Rulindil and/or Gissur, you will have to fight them later (when exiting the Embassy, and in a follow-up quest in the Ratway, respectively). It's actually possible to kill Gissur during his conversation with Rulindil without the latter becoming hostile (as long as you don't step into the elf's line of sight). It's also possible to sneak past both them while they talk and go down the stairs but, if you do that, Rulindil might magically appear in front of you (although he's supposedly still in his office talking to Gissur). All things considered, it's probably best to kill both of them before proceeding to the dungeon.

There are some locked display cases in Rulindil's office, containing two Elven Daggers, a Gold and Ruby Circlet and a Gold and Emerald Circlet; if you neglected to have Malborn smuggle your lockpicks into the party, you might be able to recover a precious few from the Thalmor you killed. You may also walk upstairs. In one of the rooms, you'll find a Safe containing the high-valued Glass Helmet of Eminent Alteration, an expensive Gold Ruby Necklace, some gold, a Silver Ingot and a Ruby.

After recovering quest items in the offices, the quest arrow will lead you down into the dungeon, where Etienne Rarnis is being kept captive. If you killed Rulindil in his office, the only other person in the dungeon will be a guard, patrolling outside the cells. If you let Rulindil walk down the stairs, he will also be there, sitting at a desk (taking notes) while the guard tortures the prisoner.

To proceed to the next stage, you need to acquire information about Esbern. This can be done in three ways: Whichever method you choose, learning about Esbern triggers a scripted event leading to the next stage of the quest.
 * Pretend you're an interrogator and order the prisoner to repeat what he told the Thalmor.
 * Release the prisoner and ask if he has any useful information.
 * Open the chest and read the documents inside.

The escape
After you learn about Esbern, two Thalmor guards will enter the dungeon with Malborn in custody, his cover blown. The guards hold the key to the trapdoor, so you will have to kill them. While saving Malborn is possible, perhaps allowing him to die there might be a kinder fate, as he points out that the Thalmor will now hunt him for the rest of his life.

Once through the trapdoor (using the key looted from the guards), a Frost Troll will be waiting, as well as any followers you might have left before entering the party. Malborn will help you fight the troll, but Etienne will simply run for the exit. There is a safespot if you get the troll on the upper stone walkway. Stand behind the rock on the lower walkway and the troll will come to you but not attack you.

Under the stone ledge you just jumped off of, you will also find a necromancer's body, an Illusion Skill Book : Before the Ages of Man, a Stone of Barenziah, and some extra loot.

Exiting the cave will allow you to see off anyone you saved (they don't seem very interested in talking) and you can then return to Riverwood to reclaim your gear from Delphine's chest.
 * Note: The Stone is only under the ledge in Patch 1.4 or later.

If Malborn survived, he can later be found in Windhelm's Gray Quarter, where you can help him escape Skyrim by killing a Khajiit assassin (located by the stables), allowing him to make it safely (for the time being) to Morrowind. If Etienne survived, he can later be found in Riften among the Thieves Guild. However, as the Frost Troll often kills Malborn and Etienne Rarnis very easily, it might be worth going to Reeking Cave and eliminating it before boarding the cart to attend the party. Inside if you search the body of the man who interrogated Etienne you can find a scroll for fire storm. You may find it helpful to use the full form of Unrelenting Force, and the using this scroll. However, this will kill anyone else around it, ally and enemy.

Trivia

 * After leaving the embassy following this quest you may return to the gates and they can now be opened through normal lockpicking, whereas before the quest they required a key to open. However the embassy building itself (where the party was held) will be locked and requires a key.
 * You can find Thalmor Robes in a room near the kitchen which will act as a disguise, making it easier for you to sneak around. Imperials, Bretons, Nords and Redguards can avoid detection at a distance, Dunmer and Bosmer can get closer, and Altmer can stroll around without question. This only works if you have no weapons, spells, amulets/necklaces, or rings equipped, and you do not attack anyone.
 * Sometimes, a dragon attacks after speaking to Delphine at Katla's Farm, or while in the Embassy Courtyard.
 * All distraction options will have hilarious dialogue if you wait a few minutes before proceeding with the quest.
 * NPCs outside of the quest after its completion will talk about the party. At least one will mention "I heard some (the Dovahkiin's race) ruined the party."
 * If you have talked to Maven Black-Briar previously and then talk to her during the party, she will comment how you should refrain from talking to her unless you want her to blow your cover.
 * Although Ondolemar is in the party, if the Miscellaneous quest of giving him Ogmund's Amulet of Talos isn't completed before the quest, it can't be completed during the party.

Quest preparation and completion

 * Some players, upon completion of Diplomatic Immunity, do not receive the quest, A Cornered Rat, and cannot progress the main questline. Affiliation with the Thieves Guild is speculated as being an issue, but is not confirmed, as some other players affiliated with the Guild don't have the issue.
 * If you've discovered the embassy before the quest, it may not count towards the "Explorer"Achievement.
 * In some cases(Including Me), when you try to return to Delphine to receive your invitation to the party, she will become hostile and will make an attempt to kill you for unknown reasons, because of this, you won't be able to finish the quest or progress through the storyline. No causes have been found yet. *Possible cause is the completion Boethiah's Calling quest. I have loaded an old game save from just before retrieving the Ebony Armor and the npc Delphine no longer attacked me. After I got the armor, she would.

Giving and getting back items glitches

 * After speaking to Delphine before riding to the embassy, you will be frozen in one spot, unable to move or use items or skills. Also, the game may crash when retrieving your items in Riverwood with the "Take All" command. This can be solved by emptying your inventory as much as possible into a safe house chest after speaking to Malborn.
 * People who have downloaded the game on Mac through a port may find that they cannot retrieve ANY gear at all from Delphine upon completion of this quest, so put it in a chest in your home.
 * It is advised to not give money to any companion before attempting the quest as all but 1500 of it may be gone.
 * Note that when giving Delphine money for safekeeping before leaving for the Embassy (for example, all of it), when the quest is completed, you may not actually get the physical money back in your inventory. The inventory status on the bottom will indicate that you indeed still have the same amount of money you gave her, but there won't be the physical Gold item in your inventory. The gold amount on the bottom status header will still increase and decrease with any additional gold you obtain or give away.
 * If you give amethysts to Delphine and after the quest you retrieve them, they may become stolen.
 * If you have Dead Thralls when you are transported to the Thalmor Embassy, they may disappear.
 * Some items may not be returned by Malborn or Delphine. These include: Stolen gold, Dragon Masks, some heavy armor, the weapon Chillrend, and Ancient Shrouded gear. Any unique Items are considered to be at risk. There is no known fix for this.

Glitches during the quest

 * The chest where you retrieve your items may not have your items as the door leading into the room may glitch making Malborn unable to lock the door. Even restarting the quest may not help it. If it doesn't help, once you kill all the Thalmor soldiers in the first room on the left take the gear and weapons they have.
 * Sometimes Malborn spawns and appears on the map to be halfway between Ilinalta's Deep and Fort Greymoor. Since you can't continue the quest line or leave the party, the only option is to load a previous save point. It is possible to physically wait, with the game un-paused, until he makes his way to the Embassy. (Timing may vary.) The reason for this glitch is unknown, though it may be caused by having a companion or follower with you.
 * Sometimes upon entering the Embassy NPCs will not appear. It is also possible that the NPC's appear, but slide around instead of walking. Either way, Malborn cannot open the door and you must restart.
 * If you re-enter the Solar after killing the Guards draging Malborn to the dungeon and freeing both prisoners your follower may appear and ecompany you through the rest of the quest.
 * Sometimes upon clearing the dungeon in the Embassy, regardless of dialogue options with the prisoner, the event with Malborn and the soldiers entering the dungeon will not occur, and the quest will not continue.
 * This may not actually be a glitch, it could be caused by not reading the books required.
 * After grabbing your equipment and going upstairs, attempting to backstab the magician may render you into the wall. (Untested, happened once)
 * If the Dragonborn is a high elf, your character can shrink and will remain shrunk for some period of time, reload on the auto save after coming through the door to correct the issue.
 * If playing as an Altmer and in disguise, one of the guards in the first room after Malborn closes the door will always be suspicious of you, drawing his weapon as you approach. Nothing should happen so long as you keep the Hooded Thalmor Robes on and don't have any weapons equipped. (Attempting to initiate dialogue or getting really close to the guard MAY cause him and all other guards to turn hostile). No other Thalmor in the Embassy seem to be affected by this.
 * The guard in front of the Solar does not leave his post if the Dragonborn (as an Altmer and disguised) persuades him that Elenwyn is wants to see him inside. It is unknown why this occurs but does not make a major difference as everyone inside the Solar will be hostile regardless.
 * Upon speaking to the prisoner he may glitch into the wall, if so he will do this no matter what you say to him.
 * Sometimes the guards will only say "speak quickly" and will not take your invitations or let you in.
 * If a dragon attacks while you talk to Delphine at the stables, she will attack the dragon and you may be left helpless not being able to move or talk to anyone, or the dialogue will end abruptly and you must deal with the dragon in order to proceed with the quest.

Achievements
Diplomatische Immunität Дипломатическая Неприкосновенность