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 Grantebridgescire region.
The offering altar can be found in the northern part of the map, near the border with another region.
This altar requires the sacrifice of several fish. So go to the nearby water and get 10 bullheads. You can get them thanks to the rod (if you have already upgraded the settlement) or just by shooting at them with a bow. Help yourself with Odin's sight to find the right fish.