This quest begins immediately after Kingmaker. You will find Oswald by the docks, southwest of the longhouse. Go there and speak with him. A brief cutscene will play, during which Finnr will arrive and flyt with Eivor. Oswald will also join in, and you’ll have an opportunity to provide him with a suggestion to retort Finnr’s attempts. It doesn’t matter which you choose, all of them will be used.
When the flyting is done, Oswald will challenge Finnr to a holmgang, without actually knowing the details of what it means. Eivor will see this as opportunity to teach Oswald a thing or two about combat, but not in the town of Elmenham. Instead, Finnr suggests that it be done on an islet to the north. Go there and speak with him to proceed with the quest. A few dialogue options will follow, but it doesn’t matter which you choose.
After the dialogue you’ll have to beat down Oswald three times. A raid on a nearby camp filled with Rued’s warriors will then be suggested. From the islet, summon your longship if it’s not there already. When you take command, Oswald and Finnr will enter it with you. You can now sail west to the camp, and begin a raid when you reach it.
The raid will play out much like any other: you and your warriors will burn and loot as much as possible. The only potential threat will come in the form of a Skull Crusher, a dual-wielding elite unit, but you can always use abilities if you’re struggling with timing. When the Skull Crusher is dead, loot the key from the table at the southern point of the camp, and use it to open the prison cages nearby.
When all the warriors are dead, speak with Finnr to begin a cutscene, at the end of which the quest will complete. Before you leave the island camp, you’ll find a treasure chest in a tent at the southwesternmost corner of the camp. Destroy the wooden barricade to loot it for a Magister’s Cloak.