On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Asgard side quest: Njord's Lament.
Reward for completing the mission: 1200 XP.
In the place marked on the above picture (yellow frame), you will meet a lamenting Njord. You can get to the island by using a boat, which you will find in the place marked red. The man got drunk and now shouts various epithets at his Jotun wife. Take the man to the boat and paddle with him to the other shore, and the mission will end.