On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Vinland side quest: A Dead Man's Tale.
Reward for completing the mission: 1200 XP.
You will find a man's corpse in the place marked in the above picture. Next to it will be a letter in which he described the last moments of his life. At the end of the letter, he included a request that his remains be brought to Nyhofn.
Put the body of the deceased on your back and carry it to Nyhofn. Once you are there, the dead man's sister will approach you. Talk to her to complete this task.