This is a quest given by a lord of a friendly faction that is not marshall. You must be level 6+ and have 100+ renown.
Briefing[]
(Lord) gave you a fake letter to fool (faction leader) into banishing his vassal (enemy lord). You are to go near (holding), give the letter to one of your (unit) and send him into the town as a messenger, believing his orders to be genuine.
You will be asked to send one (or more) of your elite troops inside an enemy town with incriminating evidence against an enemy lord. Accepting to send your man into the town will make you lose honor. Declining the quest will decrease your relation with the lord, but increase your honor.
If you tell your soldier the truth about a trap set up in the city, you gain 1 more honor than when declining quest. This leads to quest being a failure, returning to quest giver results in another -5 Relation and +3 Honor, netting a total of -8 Relation and +6 Honor.