On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for The Old Guard side quest.
Reward: items found in Skari's house.
In the place marked in the picture above, on a small island next to Repton you will meet a man named Skari.
Sit on a bench by the fire to listen to its story. During the conversation, you will receive from the man the key to his property in Rapton (Skari's house key). Everything you find there will be yours.
Skari's house is marked on the picture above and is located near the church.
If you have trouble recognizing the right building, it looks exactly as in the picture above.
Inside, apart from valuable items, you will find a small wooden box filled to the brim with lyrics. Pick it up and take it to the man's camp.
When you are in place, put the box near Skari and again sit on a bench by the fire to complete this task.