Ethereal Entities are a form of restless being, as described by Vanessa Santoro in Keys to the Demon Prison. Ethereal entities are more a ghostly/spiritual entity rather than a physical entity like the corporeal entities.

Known Ethereal Species

• Apparitions
• Haunts
• Phantoms
• Shades
• Sky Phantoms
• Specters
• Wraiths

Source

• Rise of the Evening Star (Fablehaven #2)
• Secrets of the Dragon Sanctuary (Fablehaven #4)
• Keys to the Demon Prison (Fablehaven #5)
• Dragonwatch (Dragonwatch #1)
• Wrath of the Dragon King (Dragonwatch #2)