As you progress through the game you will reach the Resting Grounds where you can find the Dreamer Statues:
Interact with these statues for information on the Dreamers
, after which you will enter a new area where you will obtain the Dream Nail
. You will then awaken in the home of the Seer who will explain the Dream Nail to you.
The Dream Nail is used to collect essence in various ways:
1) Use the Dream Nail on Whispering Roots
to release floating essence throughout the area that can be collected. 482 total Essence can be collected this way.
2) Use the Dream Nail on a Dream Warrior
to enter a battle where victory rewards 150-250 Essence. 1100 total essence can be collected this way.
2) Use the Dream Nail on a Dream Boss
to enter a battle where victory rewards 300-400 essence. 1600 total essence can be collected this way.
Regular enemies will occasionally drop 1 essence as well.
The Seer in Resting Grounds gives rewards as you collect essence. You need to collect a total of 2400 for achievement purposes. Once you collect 2400 and talk to her, this achievement will unlock.