Skyrim:Promises to Keep

Quick Walkthrough
#Speak to Louis Letrush about Frost.
#Talk to Sibbi Black-Briar about it.
#(Optional) Tell Maven Black-Briar about the plan.
#Steal Frost's lineage papers.
#Steal Frost.
#Return to Louis Letrush.

Detailed Walkthrough
See a Man About a Horse
When visiting the Bee and Barb in Riften, a Breton bard named Louis Letrush will recruit you into doing a favor for him. Apparently, Louis has purchased a valuable horse named Frost from Sibbi Black-Briar. Unfortunately, Sibbi was locked up in the Riften Jail before the deal could be finalized. Louis asks you to discuss the situation with Sibbi in the jail, then deliver the horse and its lineage papers to him.

Possession is Nine-Tenths of the Law
At the jail, bribe or persuade the guard to be allowed in.  Visit Sibbi in his cell where he explains the whole story of how he ended up in jail. He also mentions that the horse he owes to Louis is not "actually" his; in fact, it's owned by the Black-Briar estate, namely Maven Black-Briar. However, Sibbi does not want to back out on the deal with Louis, so he offers you the second half of the payment owed if you steal Frost from Black-Briar Lodge. He also directs you to where the lineage papers are at the lodge. If you're feeling put upon, you can persuade Sibbi to give you the key to the lodge strongbox containing two-hundred fifty gold.

Horse Rustling at the Black-Briar Lodge
thumb|right|Frost located at the stables
Make your way east of Riften to the Black-Briar Lodge. The lodge is heavily guarded, and the mercenary guards will attack on sight. The lodge doors are leveled, but can be opened using the Black-Briar Lodge Key that each guard carries.

The easiest way to complete the in-lodge requirements is to kill or evade the outside mercenaries, then enter by the back door. This puts you on a level below, and mostly out of sight of, the dining room containing four mercenaries above. The reason for bothering with the upper level at all is to get the unusual gem in the master bedroom on the upper level. If your sneaking skills are high, you can just slink into the bedroom and grab the Stone. If not, or if you just want to kill all the mercenaries, your position as you come in by the back door is perfect, forcing the guards to funnel toward you down a long narrow hall, enabling you to snipe them one by one.

To the right of the ladder in the basement is the lodge strongbox. The strongbox is master level but can be opened using Sibbi's stash key if you got it from him back at the jail. Around the corner, you will find Frost's lineage papers on a table. Take the papers and leave the lodge.

Steal the horse named Frost and follow the road away from the lodge until you get close to Louis' quest marker, then cut north to meet him. There are bandits and spiders between the road and Louis, so be prepared to fight them and protect both Louis and Frost; if either dies, the quest will fail. A few well-timed save games will serve you well in case one of them dies.
For your completion conversation with Louis, there are multiple options:
Receive the leveled gold payment and to let him go with the horse. The horse, however, will inevitably die after letting Louis go... whether it be by bears or by the bandits in Fort Greenwall where Louis heads to. It is possible though to receive the payment and then immediately steal the horse; the quest will count as finished without double-crossing Louis. Note that stealing the horse will make Louis attack you.  
Persuade him to give you the horse and the payment, threatening to tell Maven about everything. If this persuade check succeeds, he will give you the money and run away.
Tell him that if he was dead, you could keep both the money and the horse. He may attack you or run away.

The latter two will grant you Frost as a horse, although it won't appear in the statistics as a "horse owned"; still, it behaves like an owned horse (helping in fights, waiting where you leave it, and so on), and more importantly, you can mount him in front of witnesses without it being considered stealing.

(Possibly only if you have already told Maven): If you give Louis the horse, or if you blackmail him out of the money, as of the most recent patch Louis remains "essential" at this point, so you cannot cut Louis down as he flees with or without Frost (presumably this is to allow for the later encounter with him and thugs, whether they are his own thugs or Maven's). If you threaten to kill him, he may fight or he may run away, but either way he will be non-essential. (Striking him is considered an assault, but with Louis himself as the only witness, so your bounty will vanish when he dies, unless he runs far enough for someone else to witness his death.)

Consequences
If you persuade Louis into letting you keep Frost, a random event may occur in which a thug appears in front of you claiming to have a "message" from Louis, and then proceeds to attack you. This event may happen more than once.
If you kill the Black Briar Mercenaries and take Frost but do not bring the horse to Louis, you will eventually be tracked down by Black-Briar thugs who have a bounty on you. Kill them and you will find a note instructing them to "deal with you" (and kill if necessary).
If you tell Maven and then deliver the horse to Louis, you may encounter a thug and Louis. The thug will kill Louis unless you intervene, and the survivor will attack you.
If you deliver the horse to Louis without telling Maven, you may encounter a thug and Louis. Again, the thug will kill Louis unless you intervene, but he will then ignore you and walk away if you don't initiate combat.

Notes
The three door locks are leveled differently. The top floor is the easiest, and the bottom floor is the hardest.
Originally, Letrush was intended to hide in the Ratway near Gian the Fist, but stages 110 and 150 and related scenes no longer exist.

Bugs
 
 
 
 
 
 
 
 
 

Quest Stages