On this page of the Assassin's Creed Valhalla guide, you'll find a detailed walkthrough of the Asgard side quest: Milk of Humankind.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will meet a cow named Audumbla, locked in small stone ruins.
Your task will be to lead the cow out of this trap. Firstly, break the wooden wall and then move the heavy boulder on the left.
Now all you have to do is smash the two wooden obstacles. If the cow refuses to move forward, destroy all the segments of the makeshift wooden gate. Follow the animal until it stops at the clearing.