On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Lady of the Lake side quest.
Reward for completing the mission: 1200 XP.
In the place marked in the picture above, near a small lake, you will meet a strange woman. When you speak to her, the door from her hut will suddenly opens.
Once inside the house, the door will shut, and the entire room will fill with toxic gas. Move the bookcase and jump out the window.
The strange woman will not show any reaction at first. As soon as you move towards her, she will start a fight. Defeat the opponent and the task will be completed.