Once you reach Asgard in Assassin’s Creed Valhalla, you meet a number of characters from Norse mythology. After you meet the Builder, he asks you to bring water from the Well of Urdr.
After reaching the Well of Urdr, you meet Tyr inside. The well is sealed, so you need to solve a puzzle to unseal it and get the water from the well. Solving the puzzle is quite easy. First, you need to climb up to the source. The source should be pointing to the sunstone just on top of the entrance. You can change the position of the source by interacting with it.
Once the source points to that sunstone, you will see the light splits into two. There are two targets which the light must reach. The first one is quite easy. The main sunstone points light to the one on its right. Light gets directed towards the sunstone near the source. Move that sunstone to point it to the target just near it, and the first target will be activated.
For the second one, you need to move the sunstone beside the target to the corner just beside the rock. It takes some time to get the exact position, but you need to point the light source to this sunstone in such a way that light hits the target after getting reflected from this. Once that’s done, the Well of Urdr will be unsealed.
Simply jump inside the well and collect the water. The way outside the well is from underneath the water, so you need to dive inside after collecting the water. Once you get out, Loki will be waiting for you outside and the next part of the quest will begin.