From the interview, an excerpt of an excerpt:
Wizards: As the title is Keep on the "Shadowfell", what can you tell us about Shadowfell?
Bruce: This is from the intro: “Realms, both wondrous and dire, border the world. One such realm is the Shadowfell. Although not inherently evil, the Shadowfell is fraught with dangers, and the barrier between worlds can be thin. Sometimes the darkness breaks into the light.”