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 East Anglia in 100%.
You will find the Offering Altar in the northern part of the map.
The offering requires 200 pieces of silver, which you can't get back. However, you will receive a new side mission that you can complete.
Information about the side missions can be found on another page of the guide.