This page of our Assassin's Creed Valhalla River Raids guide explains how to unlock the Jomsviking Hall and what can be done there.
Access to the Jomsviking Hall
If you want to access River Raids, you have to download the update. Vagn will appear in your camp. He is a new character: a wandering warrior who has come to you with his clan.
To unlock the Jomsviking Hall, you must complete the introduction. To do this, head to Vagn's boat and sail with him to a new area. Along with his clan, you will carry out several attacks. You will return to the camp after obtaining enough raw materials.
Vagn will immediately suggest creating a settlement. It will cost you 100 Foreign Supplies.
Usefulness of the hut
The best warriors can be found in the Jomsviking Hall. They will change in the course of time so you will have a choice. The more you develop your hut, the better warriors you will get and the more choices you will have.