On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough of Where Legends Are Born mission.
Sigurd is determined to return to Norway to fulfill his destiny.
Reward for completing the mission: 1700 XP.
Find and speak to Styrbjorn
Once moored, head straight to the local inn. Styrbjorn will be very happy to see his sons, but Sigurd and Eivor will not be as enthusiastic. During the conversation, you will learn that Styrbjorn submitted to the current king because he knew he could not win against Harald's forces.
Sigurd and Eivor fulfilled their duty and said goodbye to their father, Styrbjorn. When you reach the port, you can tell your brother that you have some doubts about his destination. Sigurd will reply that he knows what he is doing and that the place you're supposed to go to really exists.
After the conversation, get on the boat and sail to the northern fjord (the place marked on the world map). When you reach the location, the mission will end and you will receive 1700 XP as a reward.