Upon entering, Revek will warn you not to harm the residents of the Glade. Using the Dream Nail on Spirits is what gets you that one extra Essence you've been after. After using the Dream Nail on any spirit will result in Revek spawning and starting to continuously attack you, dealing two masks of damage in an attempt to protect the Glade's inhabitants. He cannot be defeated until you've used the Dream Nail on all eighteen Spirits.