From this page of the Assassin's Creed Valhalla guide, you will learn how to obtain Gungnir - one of the unique weapons that can be found throughout the game.
You can find and collect Gungnir (Odin's spear) during the story mission called A Brother's Keeper in Hordafylke, or return to Goinnhellir a little later. It is important to note here that you will need 4 adrenaline bars to get this spear.
Follow Sigurd to the top of the mountain to get to Goinnhellir and worship the gods. When you arrive, Sigurd will perform a ritual and open a gate leading to the heart of the mountain.
Before you follow Sigurd inside, turn around and go through the narrow gap on the right into a side cave.
The glowing Gungnir (Odin's spear) is stuck in the wall - if you have 4 adrenaline bars, you can pull it out. This is one of the best spears available in Assassin's Creed Valhalla, and its quality has already been raised to the highest level - you only need to upgrade it 3 times.