On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the side quest The Last Raid.
Rewards: 1,200 XP.
At the location marked in the image above, you'll meet a Viking working on a boat, claiming to have the best loot crew in all of Oxenefordscire. You need to help him with some preparations.
Put a little bit of ore in the barrel, prepare the woodboards on the workbench, gut the fish, and finally blow the horn (select it from the actions menu).
Several wolves will appear in the area nearby. Deal with them and then speak with the man to complete this task.