On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough of the Snotinghamscire side quest: A Cordial Invitation.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will meet a hooded Anglo-Saxon soldier who delivers a speech over the body of his fallen friend. When you talk to him, you will learn that he and his company were attacked by surprise by Picts soldiers.
After a while, Pictish warriors will swarm around you. Defeat all enemies and then speak with the hooded man to complete this quest.