On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Noble Hearts side quest in the Asgard region.
Reward for completing the mission: 1200 XP.
You will meet a noble deer in the place marked in the image above. The animal will move in a direction known only to itself, and you follow its trail. You may encounter some Jotun patrols or wildlife along the way.
As you make your way through the forest, other deers will join. Eventually, the animals will lead you to a specific location and then vanish into thin air, leaving behind a chest containing Ymir's Tear Stone. Collect treasure to complete this task.