On this page of Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Path to the Wind-Blue side quest.
Rewards: 1,200 XP.
In the place marked in the picture above, you will meet a soldier who says strange things about the dark Elves and is looking for a way to the Wind-Blue.
Take a torch and fire in a large vessel to light up the road and then follow the man. On the way, you will have to light two more bonfires.
Get in the boat and cross to the other side of the marsh with the man.
Dark Elves turn out to be just ordinary bandits. Beat ALL opponents stationed in the camp (behind the palisade too) and then talk to the man to finish the quest.