With the introduction of the River Raid game mode to Assassin's Creed Valhalla you are able to lead raids on rivers in previously unexplored areas in search for unique rewards.
To access the new content, you need to make sure that:
Your game has been updated to version 1.1.2 or above.
You completed the story arc to claim your first territory.
You built the Barracks in your settlement and recruited your own Jomsviking from Eydis.
The introductory quest A River to Raid can be started by going to the south east of your settlement and talking to Vagn.
Once you have completed A River to Raid you will be able to: