On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough of the Snotinghamscire side quest, The Myth in the Mountains.
Reward for completing the mission: 1200 XP.
In the place indicated on the image above, you will meet Pict warriors who are talking about Archibald and what happened on the top of the nearby mountain. If you want, you can also read the short note lying on the tree trunk.
Climb to the top of the mountain where the leaning tree grows, pull out a torch, and set on fire the straw soldier.
After a while, a hefty elite Pict warrior will appear. Defeat him and the mission will end.