This page of the Assassin's Creed Valhalla Dawn of Ragnarok guide contains the location and description of all the dwarven altars in the Gullnamar region.
Location: You will find this dwarven altar at the western border of the Gullnamar region.
Sacrifice: You must sacrifice 5 Hare's Feet. There are a lot of animals running around, so you can easily hunt some of them down. Remember that not every hare will yield a foot. However, if you've hunted before, you may have harems captured in England in the earlier hours of the game.
Location: You will find this dwarven altar right next to the river flowing in the middle of the region.
Sacrifice: You must offer 5 Regular Pollocks as sacrifice. You will catch them in a lake in the area.