Story-related, cannot miss.

At the end of chapter 3 - a chapter chock full of collectibles - after you've made your way to Fargrimr’s settlement and have left Fargrimr’s settlement to find the Hiddenfolk, you'll come to a large lake in a wide open clearing. After emptying the lake by solving the 3 orb puzzle nearby, you’ll be able to make your way into the land of the Hiddenfolk. Once you have done, and are making your way inside, this achievement will unlock.