This page of the Assassin's Creed: Valhalla guide includes a detailed walkthrough for the Ransacking Wenlocan quest.
Eivor weakens the British forces by stealing goods from the Wenlocan outpost. By weakening King Rhodri, he hopes to force him to start peace talks.
Rewards: 1700 XP.
Find Wenlocan outpost
The Wenlocan outpost is not marked on the world map, so to make your search a little easier we marked it on the picture above.
Steal the Briton cargo
You make the progress needed for the mission by opening up the crates with supplies. You will find them on wooden platforms on trees as well as in a cave. Access to them is defended by a whole garrison of guards.
While exploring the camp, also pay attention to the cages in which the warriors are kept, who can join your ranks. As areward for completing this mission you will receive 1 700 XP.