In Assassin's Creed Valhalla Siege of Paris, you will come across Offering Altars. On this page, you will learn where to find these altars. This will make it easier for you to max out the Evresin region.
This offering altar is located at the extreme northwest corner of the region.
To complete this side activity, you will need 3 big Brown Trouts and 3 Seal Tails.
You'll find the seals at the edge of the map, just off the shore. There is also a cairn to be stacked there.
The fish swim in the water, right next to the altar. Remember to pay attention to their size.