On the following page of our guide to Greedfall, you will find a description of A Peculiar Alliance side quest.
Where to start the quest?: Conversation with Ullan, Vignamri's leader at the end of the An Aspiring Merchant quest.
After talking to Ullan, you need to go to the Hikemet Region and talk to the Governor of Burhan. During the conversation you will receive a side quest called Attack on Caravans. Travel to the place indicated by the task and you will come across a familiar merchant who is harassed by soldiers. You can threaten them, use your charisma or intuition to save a man. Give him a trade permit, then go back to Ullan and inform him of the upcoming emissary.
Wait two hours outside. Then go inside and talk to Ullan again. Now go to Magasvar. "The Vale of the Great Battle", and enter the Vighulgsob Village. Talk to Mordun - you can let Siora clarify the matter or use charisma to convince a man of the good intentions of peace talks.
Go back to Ullan and tell him everything. You now have 6 hours to join the peace talks - head for the place and you will find that there is an ambush prepared for Murdun. Save the man and return to Ullana. Talk to a treacherous man, and you'll earn +4 reputation points for the Natives faction. Complete the secondary quest and gain 1900 XP.