Rarity and Luna are insane. Spike has been abducted by cultists. Strange pages from Luna's journal, found in the oddest of places, warn of an impending threat not even Celestia is aware of.

And in the middle of it all is the strange town of Arkham, home of the occult. Can Twilight solve the mystery of the stars, before eldritch forces tear her mind to pieces?

And just what is Concealed in the Fog?

Set six months after the events in The Muse of Madness