On the following page of our guide to Greedfall, you will find a description of Attack on Caravans side quest.
Where to start the quest?: During the side mission called A Peculiar Alliance, when you talk with Burhan.
Go talk to Captain Reinhild. The woman will direct you to the merchants who survived the slaughter. Talk to them and then go to the attack point of the caravan. Search the remains and then move to the next indicated location - kill the beasts and search the massacred human body.
Follow next markings on the map until you come across the rebels. Defeat them - just a little bit further you will find a camp with another rebel you can talk to. You can attack them or leave them alone. Whatever you choose, go back to Reinhild and talk to her. Report that the rebel case is over - if you spared their life you will get +2 reputation for the Bridge Alliance faction. Complete the quest and get 1400 XP.