This quest can be acquired from Matthew when he and Fritz have moved to the Inn in the Glade. They have a plan to gain some Groschen and exact some revenge. Speak with Fritz to find out how things stand with Andrew. You will have to follow andrew to the basement to speak in private about the plan. Persuade him that it will be worth his while and exhaust the dialogue options to find out what to do next.
Andrew will inform you that you must collect three sets of Cuman armour to use as disguises. There will be three Cuman camps highlighted on the map, where you can find Cumans to kill and loot. Once you have enough sets of armour, return to Andrew and hand them over. A cut scene will then initiate where Henry will try the armour on to see how it looks.
Andrew, Matthew and Fritz are unconvinced by the armour as a disguise. They want you to speak with a man named Milomir, who claims to have seen Cumans in close quarters. When you speak with Milomir you will discover that his tale is far from the truth. Instead, you should speak with Bran, Olena and Vashek.
When you speak with Vaskeh he will suggest that if you want to look like a real Cuman, you should hunt down a dog and skin it for its hide. If killing dogs yourself concerns you, you can purchase a dog skin from a tanner. Olena will suggest that you get hold of some animal teeth, in the form of wolf fangs. These can be purchased from a Huntsman. Finally, Bran will suggest that real Cumans use Raven feathers to frill their appearance. You can purchase these from any herbalist in the realm.
Once you have all of the items return to Andrew at the Inn in the Glade to complete the quest.