On this page of our guide to Assassin's Creed Valhalla's you will find out where to look for the yellow longship - evidence that proves against Soma's traitor.
In the course of the Stench of Treachery mission, you will have to conduct an investigation to expose Soma's traitor. Only three of her trusted people were among the suspects: Galinn, Lif and Birna.
One of the key tips to help you discover the truth is the yellow longship abandoned somewhere in the marshes. For ease of reference, we have marked its location in the picture above.
Remember to carefully examine the abandoned ship and read the short note that is inside the vessel.