Assassin's Creed Valhalla

Obtaining Mjolnir is a multi-step process. The first thing you need to do is defeat all three Daughters of Lerion. Each one gives you a piece of Thor's armor. Each Daughter also gives you a dagger. All three daggers are used to obtain Thor's helmet. Refer to Godly Reward (30G) for obtaining these 4 pieces of Thor's armor. The fifth and final piece of Thor's armor is obtained by killing all members of the Order of the Ancients. Refer to Disorder of the Ancients (50G) for doing so. Once done, return all 45 Order medallions to Hytham at your settlement to get the final piece of Thor's armor - Thor's Cape.
 
Once you have all 5 pieces of Thor's armor, equip them all and head to Norway, specifically Hordafylke. Just northeast of the "e" in Hordafylke on your map is where you'll find Mjolnir. Sail your ship up the water heading northeast. Look to your left (north) for a path among the mountains. Hop off your ship and follow it a bit to locate Mjolnir glowing in a rock. If you're wearing all 5 pieces of Thor's armor, you can interact with it to take it, unlocking this achievement. For a quick video reference of where exactly Mjolnir is, refer to the following video (specifically, at 1:48).