This page details how to start the Dawn of Ragnarök DLC for Assassin’s Creed Valhalla. Includes where to go to begin the first quest by reflecting under the tree and beginning the first vision.
Dawn of Ragnarök - How to Enter Svartalfheim
Once you have the Dawn of Ragnarök DLC, it isn’t immediately obvious how and where to begin it. You won’t be visiting Valka just yet, as you have for most of your adventures as Havi (Odin). Instead, you’ll need to rest under a tree in the southeast of Ravensthorpe, thereby triggering a cutscene. This will lead into the first quest, The Rescue, and once you’ve done that you’ll awaken again as Eivor. Only at this point will you be prompted to visit Valka, before returning to Svartalfheim as Havi.