This story is a sequel to Eveningray-Tempest of the Century

After a year of following clues, Eveningray (the royal sisters' former Professor of Runic Magic) finally finds demon Whisperer in the Castle of the Two Sisters. But the demon is much stronger than she expected and the beast takes over her power. In addition, Whisperer learns how to open a gate of the underworld in the Kingdom of the Midnight Sun and free all other demons. Celestia and Luna must break the demon's spell over Eveningray, in order to save Equestria from her frost, and stop the demon from opening the gate.