From publisher blurb:
Hidden between the Plane of Shadow and the Material, is a mysterious realm of prophecy and divination wrapped within a forest of light-devouring trees. For untold centuries it has contained a secret, locked away by the enigmatic goddess of light and darkness that wove the very demiplane into being. Terrifying creatures, from cruel kytons to dragons of shadow and wrath, invade this interstitial realm in the hopes of uncovering the goddesses' lost knowledge. Only those who can survive the midnight labyrinth have a hope of uncovering the secrets of the Twilight Demesne and preventing sinister forces from discovering the truth... whatever it may be.
Demiplanes are short supplements designed to present the time-crunched GM with an entire demiplane ready to easily insert into virtually any home campaign. Perfect as an adventure locale, the lair of a powerful ally or merely somewhere to occasionally visit for any one of a hundred reason, Demiplanes are the perfect resource for GMs looking for something a little bit different for their campaigns.