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 Paris.
Le Vieil Arbre
You will find the Offering Altar in the northern part of the region, east of the sync point.
To complete this side activity you need 2 animal hearts and 100 silver. You can find both of these things by exploring the world and hunting. You should have them in your inventory.