Promises to Keep

It is given by Louis Letrush in Riften at the Bee and Barb.

Objectives

 * 1) Speak to Sibbi Black-Briar
 * 2) (optional) Tell Maven Black-Briar about the plan
 * 3) Steal Frost's lineage papers
 * 4) (optional) steal the lodge stash
 * 5) Steal Frost
 * 6) Return to Louis Letrush

Speak to Sibbi Black-Briar
After obtaining the quest from Louis Letrush, go to the Riften Jail and either persuade the jail guard to give access or bribe him with gold. During a conversation with Sibbi, he will say that the horse wasn't actually his, but belongs to his family, and will tell where to retrieve the papers, which are at the Black-Briar Lodge. Sibbi will also give the key to the Lodge Stash (upon doing so, the optional objective to "Steal the Lodge Stash" will be added).

One option is to  go to Maven Black-Briar  and report   Sibbi's plans to her, and she will say to continue the plan and that she will take care of everything else.

Stealing Frost's Lineage Papers


When arriving to the Black-Briar Lodge, there will be two to three mercenary guards outside in front of the lodge; an archer and a swordsman. One option is to approach from the front and fight them as to enter through the front, or, sneak around and open the basement door on the west side of the house. If a third mercenary does not join the fight outside from the basement, there may be two in the basement area.

Alternately, the building can be approached from the rear. The Lodge is set back against a hill that runs along the west side of the building and out toward the road to the south.

The sneak skill can also be put to good use here, as there is a small path on the east side of the hill mentioned above. Behind the Lodge there is a wooden lookout tower and a single guard that walks between the two buildings. Kill the guard, take the key and enter from the back door (north side - top building level). No matter what approach is taken, the objective is to steal Frosts' identity papers, so the Black-Briar Lodge must be entered. The lodge is a three level building; there are living quarters and rooms above, a dining area and rooms at mid-level and storage rooms and living quarters in the basement level. All outside guards appear to carry keys to unlock the doors of the Lodge. A single guard may be in the upper level and three to four guards in the mid/dining room level. In the upstairs bedroom, there is a Stone of Barenziah on a night-table.

Once downstairs, there will be a barrel in the back with the papers in a strongbox. The papers may also be found on one of the tables.

Stealing Frost
After Frost's identity papers have been retrieved, go outside to retrieve Frost; he will be outside either in or behind the stables running around loose. Mount Frost and proceed to Letrush, who is close by.

Returning to Louis Letrush
Shortly after stealing Frost, a random Dragon encounter may occur. (Riding Frost at a very slow pace may avoid catching the dragon's attention.)  Just before reaching Louis, random leveled enemies may be present. They seem to specifically target Frost, and may also engage Louis in combat, preventing the completion of the quest until he has left combat and the enemies have been defeated.

After getting to Louis Letrush, initiate a conversation with him which will give several dialogue options. There are several options to complete the job Letrush has given and to finish the deal.

The first (and easiest) way is to simply accept the payment and give Frost to Letrush, completing the job Letrush has given and the quest itself.

A second option is to persuade him to give the payment and also to keep the horse. There are two sub options, and may be based on whether or not the Dragonborn has already spoken with Maven. The first option is to threaten to report to Maven unless allowed to keep the horse, the second is to threaten to kill him to keep the horse. The first results in a response from him that "You play a dangerous game" and he leaves the Dragonborn the horse and walks away. If his life is threatened, he comments, leaves the horse and runs away. In both cases the Dragonborn has the horse and got the gold as well. Louis will however send one Hired Thug in retaliation. The thug will randomly show up at a later point saying that "this is for betraying Louis Letrush" before attacking.

A third option to receive the reward is to demand the pay by intimidation in which the payment is received and the horse is given to Letrush which will result in a reward of leveled gold.

A fourth option is to just kill him, then retrieve the horse and loot the gold off his body.

Trivia

 * Once on Frost, follow the road slightly to the west of Black Briar Estate and there will be a small stream to the right. That's where to find Louis.
 * If Frost is lost after mounting but before returning, he is not the Dragonborn's horse and will not stay in the place they have left him until they go back to retrieve him. This includes his running away mid-combat. Be careful, searching for him before he is owned can be a tedious task.
 * If Frost runs away the first time he is dismounted to talk to Letrush, reload. He should stay at some point.
 * Sometimes after paying the Dragonborn, Louis will not mount Frost, but instead leave him there and start running northwest on foot. Frost can then be kept without having to pass the speech check.
 * If the Dragonborn threatened to report to Maven unless they kept Frost, Letrush will hire one thug to come kill them. The thug targets Frost if he is near by and will attack and kill the horse if Frost is close.
 * "Promises to Keep" is from the poem Stopping by Woods on a Snowy Evening, by the American poet Robert Frost.
 * Frost's great grandsire is registered as Sleipnir, a reference to Odin's mythological horse of the same name.
 * East of the location where Louis is met to deliver the horse, a dead bandit may be found lying by a pinto (brown and white) horse. The Dragonborn can ride (and keep) this horse, it will not be considered stolen. This is just a random encounter.

Bugs
If Sibbi Black-Briar is talked to before accepting Louis' proposal, there is a chance that Louis will vanish from the Bee and Barb. A quest marker will show where he normally would be standing, but Louis is nowhere to be found. Reveal the plan to Maven Black-Briar and continue the quest as normal. In order to trigger Louis' appearance, the papers must be stolen (which will be unmarked). Speaking to Maven may cause the "steal the papers" portion of the quest to be marked complete, and a notice appear that the Dragonborn has the papers but if they are not in their inventory then they must steal them from the lodge or the quest cannot be completed.
 * If Frost is dismounted too far away from Louis, Louis will say he does not see the horse and to come back when with it.
 * Louis may quit the conversation before any speech choices are displayed, draw a dagger and run off saying 'Who's there', at this point, Frost may become unmarked for stealing and may be kept like any other horse (quest will not complete- although it may be possible to find Louis again and turn it in).
 * Missing Paper marker
 * The papers are on a small table in front of the Dragonborn as they come in the side door, not in the strong box with the gold. If the money is taken first, the quest marker goes away. Sometimes the papers are on a table around the corner from the lockbox. Picking them up will return the quest to normal.
 * Duplicated Louis
 * Sometimes after finishing the quest there may appear two Louis at the stables of Riften or Whiterun. One of them can also be trapped halfway in the ground. If he is killed, he will also respawn outside the stable he was found him at.
 * Disappearing Louis
 * Possible Solution
 * There is a bug where Louis will attack the Dragonborn and if he is killed he will get back up. Frost will help attack, but Louis will keep on attacking. If he is killed and then the Dragonborn moves away he will will say things like "I heard something" and "who's there" as if he's a bandit. If Frost helps attack, he will stay where left like an owned horse but he will still have to be 'stolen' although even if Frost is stolen in front of guards they will not react nor will a bounty be added.
 * Letrush may also appear in front of wherever Frost is left for the night when the dragonborn sleeps. He will not steal Frost nor will he attack or intimidate the dragonborn. He just stands there. (This is only if threatened with going to Maven unless Frost could be kept.
 * Letrush may act as if being attacked when given Frost, despite no enemies being in the immediate area. He will run in the direction of Shor's Stone and stop at the town.
 * Sometimes Letrush will attempt to kill the Dragonborn on sight when they try to turn in the quest and will not die. Apparently this is a bug caused by acquiring a a bounty in The Rift from stealing Frost. Pay the fine then talk to Letrush and he should allow the quest's completion.

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