Skyrim:Season Unending

Quick Walkthrough
#Convince all parties to attend a truce negotiation
#*Talk to Arngeir to get the Greybeards to host the negotiations at High Hrothgar.
#*Talk to both General Tullius in Solitude and Ulfric Stormcloak in Windhelm to get them to attend.
#Return to High Hrothgar for the negotiations.
#Take your seat at the negotiating table.
#Negotiate the treaty.
#*Make a series of decisions about the terms of the truce.

Detailed Walkthrough
Triggering the Quest
This quest starts early in the concurrent quest, The Fallen, but only if the Jarl of Whiterun (Balgruuf or Vignar) insists that a truce must be negotiated before allowing Dragonsreach to be used to capture a dragon. If either side of the Civil War controls at least eight of the nine holds of Skyrim, a truce can no longer be negotiated, and this quest will automatically be bypassed.

If you wish to receive a reward for completing this quest, you must have joined either the Imperial Legion or the Stormcloaks.  Furthermore, it is best to do this quest soon after choosing a side in the civil war.  As you complete more of the Reunification of Skyrim or Liberation of Skyrim quests, it becomes more and more difficult to negotiate a treaty that favors your faction and thus elicit a reward.

Organizing a Peace Council

The Greybeards are the only group who command enough respect to broker a truce in the civil war.  Therefore your first task is to travel to High Hrothgar and convince Arngeir that you need his help to stop the war.  He is initially reluctant, but the fact that a peace council is necessary to defeat Alduin changes his mind.

You then need to convince both Ulfric (at the Palace of the Kings in Windhelm) and Tullius (at Castle Dour in Solitude) to attend the truce negotiations.  You can talk to the two leaders in either order, using the dialogue option "I have a message from the Greybeards" (if this dialogue option is missing, see the notes).  They will each provide a variety of dialogue options, depending upon the progress of the civil war and whether or not the other leader has already agreed to attend the council.  Each has one persuasion option, which is fastest way to convince them; however, any selection will eventually lead to an option confirming that they will "come to the peace council". Starting by talking to Ulfric is somewhat easier, because he can be successfully persuaded as long as your Speech skill is at least 25; Tullius requires a Speech skill of 50 to be persuaded.

When you return to High Hrothgar, Arngeir and the other Greybeards will be waiting in the main hall of High Hrothgar, where they have been welcoming everyone to the negotiations.  Besides the Greybeards, the attendees consist of:
The Imperial delegation: 
General Tullius: head of the delegation
Legate Rikke: his advisor; provides more colorful reactions to the negotiations
Jarl Elisif the Fair of Solitude: also provides more colorful reactions 
Elenwen: the Thalmor ambassador to Skyrim, and not a welcome addition to the council
The Stormcloak delegation:
Ulfric: head of the delegation
Galmar Stone-Fist: his advisor; provides more colorful reactions to the negotiations
The Jarl of Whiterun, either Balgruuf the Greater or Vignar Gray-Mane: plays only a small role; however, he is the primary beneficiary of the truce
The Blades:
Delphine
Esbern


The arrival of Delphine and Esbern provokes a small scene because of the mutual dislike between them and the Greybeards, but in the end they put aside their differences for the duration of the council.

You can talk to any of the attendees to get their various reactions to being at High Hrothgar or their expectations for the negotiations.  Once you are ready to start the council meeting, you will need to talk to Arngeir — although he will forcibly start the discussion if you avoid him for too long.  His dialogue reveals the meaning behind the quest's name: "Season unending" is the ancient Nord term for war, and thus reflects Arngeir's ongoing pessimism about the usefulness of the truce.

Once you take your seat at the table you will be unable to get up for the duration of the meeting and the negotiations will start.

The Negotiations: Overview
The end result of the negotiations is always that the two sides (the Imperial Legion and the Stormcloaks) exchange control over one or more holds — the government (most notably the governing Jarl) in any exchanged hold is replaced. Your role in the negotiations is to make a series of decisions about which holds are exchanged, and other terms of the truce. Regardless of your choices, a truce is always signed at the end of the quest, but the terms of the truce affect subsequent gameplay, and control whether or not you get a quest reward.

The holds initially controlled by each side are dependent upon your progress in the Civil War, meaning that the available options during the negotiations can vary substantially. The primary factor influencing the quest is which side you have joined (if any) in the Civil War; this alters the overall options, as well causing more subtle changes to various dialogues. Therefore, the exact dialogue you hear may differ slightly from any provided quotations.

Despite the variations in the quest details, the negotiations always follow a fixed overall pattern:
Pre-negotiation: Decide whether or not Elenwen can attend the negotiations.
Ulfric objects to having a Thalmor representative at the table.  You get to decide whether she stays or leaves.
Phase One: Two holds are exchanged
One side demands that they be given a major hold (The Reach or The Rift).  The requested hold is predetermined, and cannot be negotiated.
The other side requests a hold in exchange.  In most cases, you are given the choice between two possible holds (a major hold or a minor hold).
Break in negotiations: Esbern's Speech
Following phase one, one of the two sides usually balks at the terms, threatening to walk out of the negotiations.  Esbern needs to intercede with a speech reminding everyone about the threat posed by Alduin before negotiations can resume.
Phase Two: Even out the terms of the truce
A series of offers is made to appease the side that warned the congregation that they would leave.  You are given the choice of whether to accept or refuse each of the offers.  These offers continue either until the truce is balanced (neither side is favored by the overall truce) or until all options have been exhausted.
Conclusion
Arngeir reviews all the terms of the treaty, and the two sides agree to them.

Throughout these negotiations, the game maintains an internal score reflecting the currently agreed-upon terms.  A net score of zero means that the truce is balanced and does not favor either side.  A positive score means the treaty currently favors the Imperials, whereas a negative one favors the Stormcloaks.  The scores associated with each choice in the negotiation are summarized in the tables in the following sections, for the Imperial and Stormcloak variants of the quest.  Scoring System Details further explains the scoring system and the negotiation options, for those interested in a more thorough understanding of the quest.

If the final score is not zero, the side favored by the treaty is pleased by the results. If you have joined that side's faction you will receive as reward one leveled enchanted weapon. If instead your side is left at a disadvantage, you are accused of betraying your side, but there are no permanent repercussions. (This was not always meant to be the case—see the Notes section for details.) If you do this quest late in the Civil War it may be impossible to negotiate a treaty that favors your faction and therefore impossible to receive the reward.

Allied with the Imperials or Neutral
The following table summarizes the options available during negotiation if you have joined the Imperial Legion or if you have never joined either side.

In this scenario, Whiterun is considered to be controlled by the Imperials.  Therefore the aim of negotiations is for Ulfric to agree to not attack Whiterun, causing him to start negotiations by demanding that Markarth (and its hold, the Reach) be given to the Stormcloaks.


Allied with the Stormcloaks
The following table summarizes the options available during negotiation if you have started Joining the Stormcloaks.

Tullius begins negotiations by demanding that Riften (and its hold, the Rift) be given to the Imperials.



Scoring System Details
This section explains the source of the numbers and options listed in the tables in the previous two sections.  If you are simply interested in completing this quest as quickly as possible, this section probably is not of interest.

The status of the negotiations is tracked internally by a score, which ideally totals zero at the end of the discussion.  If the score is greater than zero, the treaty favors the Imperials, and the Stormcloaks will ask for more concessions (if possible).  The reverse holds if the score is negative.  Factors that contribute to the score are:
Each major hold that changes sides counts for two points.
Major holds are holds that contain a walled capital city — The Reach and The Rift are the only two that can be traded during the truce
Each minor hold that changes sides counts for one point.
Minor holds are holds whose capital city is not walled — Falkreath Hold, Hjaalmarch, The Pale, and Winterhold
Whether or not Elenwen is allowed to attend the council meeting counts for one point.
Only the Stormcloaks will be asked to pay compensation for a massacre in one of Skyrim's towns, which counts for one point.
There is recorded dialogue present in the game files. This indicates there were originally plans to make it possible for both sides to be asked to pay compensation for a massacre which could have happened at variety of possible locations. Only the alleged Stormcloak massacre at Karthwasten was actually implemented into the game upon release.

The options presented during the truce are controlled by who owns which holds in Skyrim.  The initial sides for each hold are as follows:

The ninth hold is Whiterun Hold, which is originally neutral.  It is not part of any trades in Season Unending because it is implicitly at the heart of the negotiations: the primary goal of the truce is to get the side that does not currently control Whiterun to agree to cease attacks on the hold.

The opening demand is always made by the side that is opposed to the Dragonborn's allegiance.  The demand is that they be given a major hold.  This means that the opening demand is always for The Reach (if being made by the Stormcloaks) or for The Rift (if being made by the Imperials).  Even if the civil war has started, these holds are guaranteed to have their original affiliation: the winning side is being asked to give up a hold, and the winning side has also never lost any of its original holds.

The options get more complicated with the counter-demand.  Now the side that is winning the civil war is asking the losing side to give up a hold — but the losing side may have already lost one or two of its original holds.  Normally, you get to choose between the losing side's major hold and one of its minor holds.  However, the major hold may have already been lost in the civil war, in which case it cannot be traded.  A minor hold is always available (once both minor holds have been conquered, the winning side controls eight holds and Season Unending will not start).

After phase one of the negotiations, two holds have been exchanged, and the score can range from -2 to +2.  In phase two, the game effectively tries to change the score closer to zero.  In negotiation terms, the side that is unhappy with the existing terms is demanding concessions, and is not willing to give away any more land.
Only minor holds can be traded in phase two.  Furthermore, only minor holds that have not previously been traded in the negotiations are available — the minor hold (if traded) at phase one is never offered.

The Negotiations: Details and Dialogue

Elenwen

Before Ulfric sits down at the council table, he insists that the Thalmor representative, Elenwen, be removed.  Either Tullius or Ulfric (depending upon your affiliation) will ask you to decide what should be done.  Although you may have to go through an extra round of dialogue if your decision is unpopular, your decision is always obeyed.  Having her stay makes the Imperials happier with the overall truce, but causes Galmar to nearly storm out.  Conversely, her departure makes the Stormcloaks happier, but angers Rikke.  Other than this incident, she has no real role in the negotiations.

Phase One

Even after everyone sits down, there are more delays, with a short speech from the side that is not in control of Whiterun emphasizing their reluctance to be participating in these truce negotiations.  Nevertheless, Arngeir is able to convince everyone to get down to business, at which point the opening demand is made: that a major hold (the Reach or the Rift) be given up.  The other side thinks the demand is outrageous, but eventually gives in to the request.  You have no say in this part of the negotiations — and in fact you can't do anything other than listen to everyone else.

However, when it comes time to make a counter-demand, everyone turns to you: you are asked to suggest which hold should be exchanged for the Reach/Rift.  As detailed above, there are multiple options for the counter-demand, but normally you are able to suggest either a minor hold or a major hold.  The major hold (the other of the Reach or the Rift) is always the choice that is favored by your side, and is immediately accepted. If you instead suggest the minor hold, your side will question the decision, and give you another chance to opt for the major hold.  You can change your mind, or else persuade your side to go with the minor hold.  Should your Speech skill be at least 75, you are able to convince your side that the minor hold has some small advantage.  If you fail the persuasion check you instead have to fall back on "You asked my opinion. I gave it to you," which forces a reluctant acceptance of your decision. 

The large range of possible holds under discussion means that there are many possible dialogues during this discussion.  In particular, each hold has a different set of dialogues detailing the advantages to controlling that hold.

Esbern's Speech

With an exchange of holds now on the table, the two sides of the negotiation take stock.  The side that is currently getting the worse end of the deal announces the terms are completely unacceptable and threatens to walk out.  (If the deal is even, neither side will stand up and issue this warning that they will leave, with Esbern's speech being preceded by other arguments.)

Before the entire truce is abandoned, Esbern intervenes with an impassioned plea reminding everyone about the importance of these negotiations.  In particular, he emphasizes that any soldiers killed in the war end up strengthening Alduin, because the dragon feeds on their souls. Fortunately, his speech is enough to persuade everyone to sit back down and resume the talks.

Additionally, if you allowed Elenwen to stay, she will begin to respond to the speech, but either Tullius or Ulfric will interrupt. The person who responds is determined by whoever has gotten the shorter end of the negotiations so far.

Phase Two

The side that was about to walk out is now given the chance to suggest additional truce terms to appease them and even out the overall agreement.  These options can include asking to be paid reparations (e.g., an undisclosed sum of gold) for a massacre that took place or asking to be given one or more minor holds.  In each case, you get to decide whether to agree to the additional term.

If additional terms are accepted and it evens out the treaty, then negotiations end.  Otherwise, the disgruntled side continues to complain about the unfairness of the treaty, and then come up with another suggestion.  If there are no remaining options, this unhappy faction will abruptly switch from refusing to accept the truce to instead conceding that it's the fairest deal they are likely to get.

The Completed Treaty
Once the negotiations have reached their conclusion, Arngeir will summarize all of the terms that have been agreed upon so far, and confirm those terms with each of the parties.  Tullius and Ulfric will both agree (albeit with some grumbling), and the meeting will adjourn.

The side with which you are affiliated will talk to you and tell you their opinion of the treaty.  If the treaty favors them, they will be happy and give you a reward.  If the treaty is neutral, they will be "disappointed," believing that "You gave too much away. More than was necessary to secure this truce."  If the treaty goes against them, they will say that your actions "were a disgrace" and will accuse you of betraying them at every turn. If you are an Imperial soldier, and the treaty is against them, Tullius will claim you are not loyal, saying "You swore an oath of loyalty to the Emperor, do you remember that?"  You have a choice of possible responses, but there are no repercussions, even if you respond with "I thought you needed to be taken down a notch."

Most of the parties will head back to their respective cities, but the Greybeards, the Blades, and the faction currently controlling Whiterun, the Empire if neutral will remain to talk with you about how to proceed with trapping a dragon, thus automatically leading back to the quest The Fallen. Esbern will teach you that the names of dragons are always three words of power, and inform you that while you were preparing the negotiations, he retrieved a scroll listing the names of many of the dragons slain by the Blades. Cross-checking it with Delphine's map of Dragon Burials, he discovered that the next dragon's name is Odahviing. While he won't be compelled to come when called, dragons are, naturally, arrogant creatures, and he will not ignore this challenge. More notably, your recent victory in the previous battle against Alduin will make him even more likely to respond to your Voice. Esbern will then teach you the shout to call Odahviing. In addition, Delphine will walk up and demand that you kill Paarthurnax, if the quest hasn't already been given. 

Once Season Unending is complete, the Civil War will be on hold until the Main Quest is complete.

Notes
When organizing the peace council, the dialogue option "I have a message from the Greybeards" may be missing from Ulfric and/or Tullius.  This was done intentionally by the developers to prevent various quest conflicts, and generally means that you need to progress further in the Civil War.  Some specific circumstances where it happens are:
If you have not yet joined either the Imperial Legion or the Stormcloaks, and you have not talked to Ulfric and/or Tullius since Helgen, you will be forced to first talk to them about Helgen, at which point they will suggest that you join their side of the conflict. To proceed, simply finish and exit the first dialogue, then start a new discussion, at which point the option will appear.
If you joined the Stormcloaks, you cannot continue with Season Unending until The Jagged Crown is complete.  This is necessary to prevent the possibility of changing allegiance from Stormcloaks to Imperials after completing Season Unending.  If this is the case, complete The Jagged Crown. As of the latest Unofficial Skyrim Patch (Legendary or Special Edition), the same appears to be the case if you have started the Jagged Crown quest for the Imperials: the quest needs to have either been completed, or not started, no matter which side you start it for, or complete it for.
If you have been given the quest Joining the Legion by Legate Rikke, Tullius will refuse to speak to you until you complete her assignment. Similarly, Ulfric will refuse to speak to you if you have been given the quest Joining the Stormcloaks by Galmar Stone-Fist, until you complete his assignment.
During Message to Whiterun (Imperial or Stormcloaks) and Battle for Whiterun (Imperial or Stormcloaks), progress in Season Unending is intentionally disabled because a truce is impossible while Whiterun is actively under attack.  If this is the case, complete the Battle for Whiterun before trying to proceed with Season Unending.
If completing the Battle for Whiterun after starting Season Unending causes Vignar Gray-Mane to replace Balgruuf as Jarl, Vignar will simply appear in Season Unending in place of Balgruuf — the quest proceeds identically, even though Vignar was not part of the original agreement.
If you have a talkative follower, they may continue to talk to you during the negotiations, making it hard to follow what is going on. Take them outside the conference chamber and ask them to wait before you take your seat.
You will skip this quest altogether if you've already completed The Battle for Windhelm/Solitude. 
The developers considered several other options for this quest that were not kept in the final version of the quest.  Most notably, there are options to have you kicked out of your chosen army if the negotiated truce favors the other side.  You would then be able to switch to the other side.
After the truce has been negotiated, Delphine will attempt to talk to you and tell you to kill Paarthurnax.
:* It is possible to bypass this entirely, however, if you leave High Hrothgar quickly enough.
This quest may make it impossible to get the War Hero achievement.  You may want to wait until you've completed The Battle for Fort Greenwall (Imperial) or The Battle for Fort Sungard (Stormcloaks) in order to be sure you get this achievement.
:* If you complete this quest before completing the Civil War quest line, it is safest to have the Rift (Riften) and the Reach (Markarth) controlled by opposing factions. Ensuring the Imperials take Riften and the Stormcloaks take Markarth will mean that one of the holds is always available to take by your chosen faction and the trophy will unlock without an issue from either side. 
:* There is an alternative method to obtaining this achievement as well as this quest, but you have to join the Stormcloaks in order for this to work.  First, finish the quest Joining the Stormcloaks, but do not start the quest The Jagged Crown.  Without that quest being active, you can then schedule the peace conference.  Because you are technically allied with the Stormcloaks, Tullius will make the opening demand for Riften.  When Ulfric demands something in return, give him Morthal rather than Markarth.  This will allow the Reach to remain open for Stormcloak invasion when you resume the Civil War quest.
This quest allows you to skip the normal prerequisite quests for becoming Thane for any holds that change hands during this quest, regardless of what side you're on, with the exception of The Rift. You will be allowed by the new jarl to buy property in holds that change hands if the hold is one that has property available, including Hearthfire properties if you have Hearthfire installed. You will still have to initiate the Thane quest with the new jarl, and help either 3 or 5 citizens of that hold as well as buying property in holds where that is a requirement in order to become Thane, as per the normal requirements. You will only be allowed to buy Honeyside if Maven Black-Briar is installed in The Rift; you will not be able to skip the normal requirements in that hold to be made Thane. 
If you have started the quest The Forsworn Conspiracy, but not completed it, handing over the Reach to the Stormcloaks in the negotiations (which always happens if you are Imperial or unaligned) will end the quest. If you have completed that quest but have not started No One Escapes Cidhna Mine, then handing over the Reach will remove your bounty in the Reach and prevent you from starting the Cidhna Mine quest.

Bugs
 
Since it is not possible to stand up during the course of the negotiations, one solution is to try   and wait for responses from the others at the table (nodding, etc) then  . This skips Esbern's speech, and allows you to continue with the negotiations. If this does not work, moving the quest stage forward to 300 appears to be the only viable solution. This bug should not occur if you have unpacked his voice files in the way outlined on the page for A Cornered Rat.
 
 Using   and   on Galmar Stone-Fist may fix this issue. Standing further into the room than Ulfric and using   on Galmar may also work.
Speaking to the participants in the Peace Council, 'moving' them into the Council Room (either via the console or by using Unrelenting Force), or waiting for 24 hours may fix this. If not, just reload a previous save.
 
Completing Dragonslayer seems to cause everyone affected to return to their normal schedule.
 
Liberating at least one hold in Liberation of Skyrim seems to resolve the issue.
 
Before version 1.4 of the Official Skyrim Patch, there were some serious bugs that would affect your ability to complete the peace treaty.
Without the patch, it is best to complete Season Unending before starting any of the Civil War quests. This means you must refrain from talking to either party about joining their army. As soon as you do and confirm you want to join, you have started that quest line.
Most other bugs can be resolved by completing one or more Civil War quests — either through to the end of The Jagged Crown (Imperial or Stormcloaks), or to the end of Battle for Whiterun (Imperial or Stormcloaks).

Quest Stages