Assassin's Creed Valhalla

This achievement requires quite a bit.
 
First, you'll need two specific powers equipped: Power of Jotunheim, which lets you teleport, and Power of the Raven, which lets you turn into a raven. The former can be found on a number of different Jotnar enemies (prevalent in the southwest and central areas); the latter can be found by killing any of the giant raven enemies around the world. 
 
Next, you'll need a couple upgrades. You'll need the Jotunheim Assassin upgrade for the Power of Jotunheim, which grants the ability to teleport assassinate while that power is active. You'll also need to buy the center Hugr-Rip upgrade which gives you a second Hugr bar. 
 
Now, with all of that set up, you need to find a restricted area with enemies. An easy area is right east of the viewpoint in Eitri in the center. You'll want to identify two enemies who are not close to each other or not visible to each other, nor are they near any other enemies. You will also need both Hugr bars full and two full bars of adrenaline. 
 
Now comes the execution, and this is important to get exactly right or the achievement won't unlock:
 

1. Trigger your Power of Jotunheim
2. Aim at the first enemy with LT and press RT when prompted to teleport assassinate. 
3. Move toward and within view of the second enemy while you wait for your stamina bar to refill
4. Aim at the second enemy with LT and press RT when prompted to teleport assassinate
5. When the assassination animation is done, trigger your Power of the Raven ability to turn into a raven
6. Fly until you are out of the restricted zone and land anywhere outside the zone, not near enemies
7. As soon as you land this achievement should unlock, if you did everything correctly. 

The key thing here is flying far enough away to leave the restricted area before you land. Fly even farther if you want to be safe. 

Refer to the following video to see these steps put into action.