This prequel is narrated by Alice Grimm.
Immortals are the fairy tale characters themselves. They cannot be killed, but if they are killed in Dreamworld it puts them in a coma they will not wake from. Dreamhunters are the individuals who can enter an Immortals dreams to kill them.
In this story, Alice flies to Venice on a mission to “find Cinderella” with the help of Loki, a Dreamhunter.
Bella shows her the corpse of a witch that was burned, with 7 nails in her jaw, and staked to the ground. When the tarp is removed, Alice learns that the witch was buried with 17 glass slippers. Making this witch the individual who can help her find Cinderella.
During the time spent in Dreamworld, Alice tries to explain a lot of this to Loki. They begin looking for the witch, which is unusual for Loki but he agrees to help Alice complete her mission.
This story gives you a bit about who Loki is. Which is suppose to help out later in Snow White Sorrow.