I Believe

Elsa doesn't believe in legends like Anna does. That's how she loses her kingdom to the Boogie Man; or better name, Pitch Black.

The whole of Arendelle was covered in black nightmare sand, blocking out the light of the sun.

Every occupant has been turned into a new type of nightmare soldier; even Kristoff and Elsa.
The only survivor is Anna, who must keep her beliefs close because that is the only thing shielding her from the nightmares and nightmare sand.

"The legends are real, maybe they can save us"