The fall down to the Resting Grounds is in the bottom right of Crystal Peak. > The Lumafly Lantern is recommended because getting to the pit leading down to Resting Grounds requires traversing one or two dark rooms which need the lantern so you can see where you're going.
There are several first-time access points to the Resting Grounds which require different amounts of progress:
In order to start collecting essence, you need to acquire the Dream Nail which can be found in the Resting Grounds.