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 max out the Essexe region.
The offering altar can be found in the northern part of the map, near the border with another region.
In order to complete this activity, you will need 50 pieces of iron ore. You don't need to kill or chase anything, collecting the resource will be enough.