The next section of the Greedfall Guide contains a walkthrough of the side missions The Price of Blood.
Where to start the quest?: After the side mission Logging Expedition.
Go to the district Kunwar and find an abandoned mine. It turns out that it is guarded by soldiers and you can not go inside. To the right of the entrance, you will find a passage - look at the situation in the mine from above.
Now go to New Serene and talk with Sir de Courcillon. Go to Lady Morange, and then find and check the certificate of ownership. The next step is to meet with Millard and return to Sir de Courcillon to sign a new deal.
After that, you can go back to the mine - show the new documents at the entrance, and then go inside. Defeat a bunch of opponents and then rescue the slaves from the prison and the cage. Now go to Dunncas and tell him everything.
This concludes the quest - you will get 1900 XP and you will a +4 increase of reputation with the Natives.