In Assassin's Creed Valhalla, you can find Offering Altars. On this page of the guide, you will learn where to find the altars. This will make it easier for you to complete Asgard in 100%.
You will find the Offering Altar in the northern part of the map.
In order to complete this activity, you must collect all Stone Tears available in Asgard. They aren't used for anything else. This isn't a troublesome task. You will easily reach most of them, and others are obtainable during side quests.
You will receive 5 skill points for activating this altar.