Diplomatic Immunity

Quick Walkthrough

 * 1) Meet Delphine in Riverwood.
 * 2) Meet Malborn in Solitude.
 * 3) The Dragonborn gives Malborn their equipment.
 * 4) Meet Delphine at Katla's Farm in Solitude.
 * 5) Create a distraction and get away from the party.
 * 6) (Optional) Retrieve the Dragonborn's gear.
 * 7) Search for information about the Dragons returning.
 * 8) Escape the Thalmor Embassy.
 * 9) 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.''

Meeting Malborn
Delphine thinks that the Thalmor know something about the dragons' return to Skyrim. She has a plan to get the Dragonborn into the Thalmor Embassy, to gain access to their secret documents in an attempt to find out more about the situation at hand. She will acquire a legitimate invitation to a reception being held by the Thalmor ambassador, Elenwen. Find the Bosmer servant and spy for the Blades (Skyrim), Malborn, at the Winking Skeever tavern in Solitude.

When arriving, Malborn will agree to smuggle some equipment into the back-rooms. Malborn can bring an unlimited amount of gear with him, while the only gear the Dragonborn will have during the beginning of the mission will be what has been given to Malborn.

There is only one chance to give him gear. Nothing else can be given him once the trade dialogue closes, and remember nothing else can be taken inside; give it to Malborn. Give him some lockpicks, since there will be locks to pick.

Afterwards, the Dragonborn will meet with Delphine at the Solitude Stables at Katla's Farm to pick up the invitation and party disguise, and then drop off everything else on hand with Delphine; everything given to her will be stored safely until reclaimed after the mission. Delphine will store the inventory in a chest in her hidden office in the Sleeping Giant Inn, in Riverwood.

Once at the party, the Dragonborn will be approached by ambassador Elenwen and will become engaged in conversation. Malborn interrupts the conversation, causing the ambassador to walk off. The Dragonborn then has the opportunity to speak with Malborn and ask him for a drink.

Distract the Party Guests
Ambassador Elenwen greets the Dragonborn upon entering the Thalmor Embassy. After speaking with her, the Dragonborn will need to create an opening to slip out of the party; tell Malborn when ready, and he'll wait by the door behind the bar for the Dragonborn's next move:
 * If Razelan is talked to, (he's on the bench nearest to Malborn) he'll ask for a drink. Malborn can conveniently provide the Dragonborn with a Colovian Brandy to give to Razelan, who then offers to do the Dragonborn a favor in return. Getting a drink from the Bosmer servant, Brelas, and giving it to Razelan will also work. Tell him to cause a scene, and he will make a grandiose announcement, causing a distraction.
 * Alternatively, if the Dragonborn is acquainted with Jarl Idgrod Ravencrone and has previously assisted her, she may be asked to cause a distraction, as "an old woman can get away with anything". She will then begin harassing Razelan and making a scene, providing the necessary cover.
 * Another option is to overhear Erikur trying to flirt with the Bosmer servant Brelas, unsuccessfully. Speaking with Erikur afterward will make him mention his weakness for Bosmer women, and the Dragonborn can therefore offer to speak with her on his behalf. Speaking to Brelas reveals her dislike for guests like Erikur and asks that he be persuaded 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.
 * If on good terms with Ondolemar, the Dragonborn may ask him to make a distraction (after a persuasion check) where he begins accusing Razelan of insulting the Thalmor.
 * If the Dragonborn has a higher speechcraft skill and Vittoria Vici is there, she can be persuaded to cause a distraction.
 * If the quest for Orthus Endario has been completed, the Dragonborn may ask him to make a distraction, in which he will accuse Razelan of insulting the Thalmor.
 * If the Dragonborn has become the leader of the Thieves Guild, then the Dragonborn may request Maven Black-Briar to cause a distraction.
 * Other people the Dragonborn has done favors for might be in attendance, including other jarls.  Some might be persuaded to make a distraction.  This distraction, like that caused by Jarl Idgrod Ravencrone, will likely involve making a scene at Razelan's expense.  This has been observed with Igmund and Elisif the Fair, among others.

Once there's an opening, go to the door behind the bar and follow Malborn. Go through the kitchen to a pantry, retrieve the smuggled equipment from a chest, and go through a door into the next hallway. Malborn will lock the door behind the Dragonborn, completing this phase of the mission.

Thalmor espionage
As Malborn locks the door, look through a doorway to the left and overhear two Thalmor agents chatting at 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 or sneak around the mage, and then turn left again to enter a room with access to a second floor exit. This will open 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 he is attacked. 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).

A Dragonborn that is an Altmer and is wearing hooded Thalmor robes may walk past all the guards without incident. Walking is essential, as immediately running, even as an Altmer and wearing Thalmor robes will cause the guards to become hostile. Even if the Dragonborn is an Altmer, the Thalmor wizard at the door to the Solar will ask why they are not at their post. At this point, the option to persuade the wizard that Elenwyn wants to see them inside becomes available. If this persuasion is successful, the wizard will say to "take his post" and say not to let anyone in, at which point he should leave the area but appears to stay where he is. Either way, once inside the Solar, any disguise will no longer work and everyone inside will become hostile on sight.

Inside, either kill everyone or sneak over to the chest the quest arrow is pointing to, and grab the needed items. Read the documents taken to activate the next arrow. If Rulindil and/or Gissur are not killed, they will have to be fought later (when exiting the Embassy, and in a follow-up quest in The Ratway, respectively).

There are locked display cases in Rulindil's office, containing two elven daggers, a gold and ruby circlet, and a gold and emerald circlet.

After recovering the quest items from the offices, the quest arrow will lead down into the dungeon, where Etienne Rarnis is being kept captive. If Rulindil was killed in his office, the only other person in the dungeon will be a guard patrolling outside the cells. If Rulindil was allowed to walk down the stairs, he will also be there, sitting at a desk (taking notes) while the guard tortures the prisoner. Information about Esbern is needed to proceed. This can be done in three ways: Whichever method is used, learning about Esbern triggers a scripted event leading to the next stage of the quest.
 * Pretend to be 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 the Dragonborn learns 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 they have to be killed or it can be pick-pocketed from one walking pass by while hiding behind the doorway that is located behind and left from the Rulindil. If the Dragonborn is an Altmer dressed in full Elven Armor the guards will not be hostile, allowing them to get close enough for melee weapons to be effective.

Once through the trapdoor, a frost troll will be waiting, as well as any followers that might have been left before entering the party. Under the stone ledge is a necromancer's body, an Illusion Skill Book: Before the Ages of Man, a Stone of Barenziah, and some loot.

Exiting the cave will allow seeing off anyone that was saved (they don't seem very interested in talking). Return to Riverwood to reclaim any gear from Delphine's chest. If Etienne survived, he can later be found in Riften among the Thieves Guild.
 * The Stone is only under the ledge in Patch 1.4 or later.

Trivia

 * It is not possible to go into sneak when the party is first entered. Sneaking becomes possible only after Malborn has opened the door allowing the Dragonborn to leave the party.
 * If Malborn escapes, he can later be found in Windhelm, in the Gray Quarter bar. He offers a quest to Find the Thalmor Assassin that the Thalmor sent after him for his treachery.
 * If the embassy is returned to following this quest, the gates will be able to be opened through normal lockpicking, whereas before the quest they required a key to open. However, the embassy building itself will be locked and requires a key.
 * The wizard found after exiting the kitchen has Hooded Thalmor Robes which will act as a disguise, making it easier 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 no weapons, spells, amulets/necklaces, or rings are equipped. It does not work for Khajiit, Orsimer, and Argonians regardless of any other circumstances.
 * A dragon may attack the courtyard and be beneficial in distracting the Thalmor soldiers. However, it may appear immediately after exiting the cave, thus adding a considerable challenge in keeping Malborn, Ettienne and Brelas alive (they will attack it unarmed).
 * Regardless of race (even if an Altmer), if a vampire, no one will believe the disguise and will attack on sight.
 * Characters outside of the quest will talk about the party. At least one will mention "I heard some (the Dovahkiin's race) ruined the party."
 * If Maven Black-Briar has been talked to previously and is talked to during the party, she will say to refrain from talking to her unless the Dragonborn wants her to blow their cover. (She does this even if she hasn't been spoken to before.)
 * 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.
 * People who provide distraction options (Aside from Razelan himself and Erikur) will always make a scene about humiliating Razelan.
 * A dragon may appear right after giving the Dragonborn's equipment to Delphine. This may prove a challenge as they will have no weapons or armour - aside from quest items (e.g. Ulfric's War Axe) and cannot speak to Delphine and board the carriage for the party until the dragon is dealt with.
 * It is possible to sneak in a small amount of equipment without Malborn. Before giving Delphine the equipment, go to the Thalmor Embassy and place anything desired on top of the wall next to the main gate (there is an invisible wall preventing throwing it over). Go to Dephine and board the carriage to the embassy, and the gear should be on the outer wall (be careful with valuable items as they may fall off). Tested on.
 * You're able to hear gossip about the party around Skyrim.
 * You can heard Jarl Idgrod Ravencrone and Maven Black-Briar talking about the Grey Quarter and rumors that Ulfric Stormcloak was slain when defending his city against a dragon. Maven humoursly said that Ulfric himself started the rumor to get more recruits.
 * You can also hear Idgrod and Erikur speaking about what made Rorikstead a great beautiful harvest.
 * You can also hear Idgrod and Maven speak about Khajiit Caravans camps outside the walls of the city and that Maven speaks highly of them for their courage in the war and dragons while insulting the Stormcloaks by being an armed mob.
 * If a player has previously taken advantage of the Fortify Restoration glitch and enchanted a piece of armor with a Fortify Health enchantment the quest will become impossible because the Dragonborn will die upon removing this item. The fix for this requires that you make a ring (or necklace) with the same (or similar strength) enchantment as you are allowed to the party with such items.
 * It is possible to begin this quest by wall glitching into the embassy and stealing the papers.

Bugs
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Quest preparation and completion

 * Sometimes, upon completion of Diplomatic Immunity, the quest, A Cornered Rat, does not trigger and the main quest-line cannot progress.
 * If the embassy is discovered before the quest, it may not count towards the "Explorer"Achievement.
 * Dwarven Armor, Elven Armor and Dragon Priest Masks may disappear if given to Malborn.
 * In some cases, when trying to return to Delphine to receive the invitation to the party, she will become hostile and will make an attempt to kill the Dragonborn. This will make the quest unable to be completed, and the Dragonborn will not be able to proceed in the storyline. ( Sometimes, completing other quests and then returning to Delphine may fix this bug.)
 * If a Dragon has attacked and killed the exterior guards at the embassy, upon taking the carriage to the embassy party the game may freeze. This can prevent the Dragonborn from completing the quest normally.
 * Solution: Using a two-handed weapon to hammer Shadowmere or Frost through the main gate so the horse can then be mounted as a means of teleporting through the gate. Once through the initial gate the player must then glitch themselves over the fence separating the main gates from the Solar building, this can be accomplished by riding toward the left side of the building that would've been entered for the party and then leaping at the wall with the short fence section atop it, done correctly the horse will glide upward along an invisible rail and over the top of the fence and into the courtyard beyond. Alternatively, with Arvak, persistence and trial-and-error you can actually jump over the walls on the left side of the embassy by continually jumping into and alongside the wall while trying to make your way to the back of the complex. Once this is accomplished, enthter the unlocked back door of              the party building and proceed through the building (no enemies will be present) and to the chest the Dragonborn would have normally retrieved their items from, this triggers the remainder of the quest (including spawning the enemies not present on the way in from the back) as it would have normally occurred had the party section not been disrupted by a random dragon attack killing the guards.
 * If the quest is repaired in the above fashion, the Unusual Gem at this location may not spawn, however one of the gems previously acquired (most often the one in the Riften palace) will respawn, still allowing No Stone Unturned to be completed.
 * If an Altmer wearing Thalmor armor, may be treated as a Thalmor.
 * If the Dragonborn have been messing around with the fortify restoration enchanting loop and topped the HP cap (i.e. anything over 10 million HP), this quest is impossible to finish by usual means. The player will die instantly when the enchanted object is removed, and the quest-specific party gear cannot be enchanted.
 * If Delphine becomes hostile, when returning to Solitude Stables, then reload a save from before advancing the quest at the Winking Skeever in Solitude and complete the quests "Bound Until Death" and/or "A Daedras best friend". (I completed "Innocence Lost" questline)
 * If the Dragonborn is using a weapon/spell to make enemies flee when attacked, the two guards escorting Malborn can flee into the trapdoor and since one of them has the key to opening said trapdoor, it is impossible to complete the quest. When closing the other door it may require a key

Giving and getting back items glitches

 * After speaking to Delphine before riding to the embassy, it is possible to be frozen in one spot, unable to move or use items or skills. Also, the game may crash when retrieving the items in Riverwood with the "Take All" command. This can be solved by emptying the Dragonborn's 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.
 * It is advised to not give money to any companion before attempting the quest as all but 1,500 of it may be gone.
 * If Delphine is given money for safekeeping before leaving for the Embassy (for example, all of it), when the quest is completed, it may not actually be gotten back. The inventory status on the bottom will indicate that the same amount of money given her is back, but the physical Gold item won't be in the inventory. The gold amount on the bottom status header will still increase and decrease with any additional gold obtained or spent.
 * If Amethysts are given to Delphine and retrieved after the quest, they may become stolen.
 * Dead Thralls may disappear when the Dragonborn is transported to the Thalmor Embassy.
 * Some items may not be returned by Malborn or Delphine. These include: Stolen gold, Dragon Masks, some heavy armor, the weapon Chillrend, any Elder Scroll and Ancient Shrouded gear. Any unique Items are considered to be at risk. There is no known fix for this.
 * If you leave arrows with Delphine for safekeeping, they may be marked as "stolen" when you try to retrieve them

Glitches during the quest

 * Even after the Thalmor Guard accepts the Dragonborn's and Razelan's invitations, the Thalmor inside the embassy may become hostile. This will cause Razelan to flee and run around outside, making the quest unable to be completed if a distraction cannot be caused by other means.
 * The chest with the items may not have them as the door leading into the room may glitch making Malborn unable to lock the door. Even restarting the quest may not help it.
 * When talking to Delphine before leaving you may not be given the Party clothes and when entering the carriage, the screen may go black and you may not travel or be able to move, attack etc.
 * Sometimes Malborn spawns and appears on the map to be halfway between Ilinalta's Deep and Fort Greymoor. Since the Dragonborn can't complete the quest 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.
 * 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 the quest must be restarted.
 * If the Dragonborn re-enters the Solar after killing the guards that were dragging Malborn to the dungeon and freeing both prisoners, their follower may appear and accompany them 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 taking the Books in sneak mode and killing Ruindil, the Thalmor and Malborn appeared and killed Malborn at the wooden Stairs which are leading to the cell.

If the Dragonborn is a High Elf, they may shrink and remain shrunk for some period of time, reload on the auto save after coming through the door to correct the issue. If a Werewolf, transform and stay in beast form for a while then change back. This works for some glitches as well.
 * 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, drawing his weapon as the Dragonborn approaches. Nothing should happen so long as the Hooded Thalmor Robes are kept on and no weapons are 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.
 * It is possible to become trapped in the embassy. When Malborn tells the player to cause a distraction, if the player waits too long during said distraction, the guards will return their posts and Malborn will return to the bar and close the door behind him, locking the player in the small hallway beyond if they try to head back there and do not come out before the door closes. Both doors out of the hallway require keys to open, and the player will not have either.
 * The guard in front of the Solar does not leave his post if the Dragonborn (as an Altmer and disguised) persuades him that Elenwyn 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 is said to him.
 * Sometimes the guards will only say "speak quickly" and will not take the group's invitations or let them in.
 * }