Not everyone always remain your ally in Assassin’s Creed Valhalla. When you’re working your way through The Song of Soma in the Grantebridgescire region, after you regain Grantebridge from The Order of the Ancients, Soma pulls Eivor aside. Sheh wants to confide in them that the way The Order of the Ancients were able to overrun the town was because they found out about the tunnel. Only her, and her most trusted advisors, Galinn, Lif, and Birna, knew about it. Soma asks Eivor to find out which of her advisors it was.
When you jump down into the tunnels and walk through them, you discover yellow paint. The traitor had placed a yellow cross on the rock near the entrance to show the attackers where to go and then dashed off away from the crime scene. The traitor left piles of yellow behind, and you can use Eivor’s Odin Vision to follow them to the shoreline. However, when you reach the water, there’s no more paint trail. Eivor summarizes that the traitor had left, painted their longship yellow, and sailed down the river.
To find the yellow painted longship can take a little bit of time. It’s a good idea to start at Grantebridge and go to the northeast, up the Great Ouse River. Keep traveling north until you reach Utbech, and if you choose to raid this location, you can find a note in the hut with the wealth point of interest that was written by Wigmund, an Order of the Ancients’ leader who led the attack on Grantebridge, that the longship with the yellow front is not to be touched, and is instead their ally. This section is optional, but it does prove you’re on the correct path.
Sail past Utbech, but when you reach the fork, go to the right side, follow the river, and then take a left. The yellow-painted longship’s location will be between those two islands. When you arrive at the longship, several enemies will be protected, but you can make short work of them.
When you go inside the longship there will be an inscription on the right side of it. Lif, Soma’s expert shipbuilder, crafts a unique poem for everyone he makes. The one inside that longship was made for Galinn, confirming it was his longship.
Now, using your Odin’s Vision, there is an interaction near the boat where Eivor can break down the scene. They describe how the longship had been attacked by bandits, and ambushed, and the traitor, Galinn, had fled the scene. That’s where you later found him and saved him. You won’t be able to immediately bring this to Soma, but you can use this later when Eivor must make the decision about the individual.