On this page of our guide for Assassin's Creed Valhalla you will find a detailed description of the passage of the mission called An Island of Eels.
Eivor helped Soma expose the traitor who sold information to the Saxons and enabled them to attack Grantebridge. It is time to face Wigmund and free Grantebridgescire from the hands of the vile Lord.
Rewards: 3400 XP.
Regroup with Soma's clan
You will meet Soma and her clan near the monastery on Island of Eels. During the conversation you will find out that her group will be on standby and if you encounter any problems along the way, they will immediately attack.
Find and assassinate Wigmund
Wigmund managed to escape from Grantebridge and hid behind the solid walls of a monastery on the Island of Eels. You can complete this task in two ways: quietly or by raiding the monastery.
You can get to the main building without bloodshed. Jump on the roof, and then enter through the open window on the right.
Wigmund does not remain in one place but walks around the interior of the building. You can lie in wait for him in the place shown in the picture above and quietly eliminate it, and the guards won't even have time to react. You don't have to fight them, you can just leave and go back to the camp. After a longer cut-scene the mission will end and you will receive 3400 XP as a reward.
If you have correctly guessed Soma's traitor in The Stench of Treachery mission, Birna will now join Eivor's crew and the Raven Clan.