Evil lurks in the castle. After their victory over Vanestra, Prince Collin and his friends believed they had freed the castle from evil forever. However, deep in the darkness of the charred vaults, something sinister stirs. A terrible power rises, putting the prince and his friends in grave danger and forever altering the fate of the city of Borkenheim.

Beautiful firefly, twinkle, twinkle! Let your light shine in the dark night, so that even in pain, love awakens. Defy sorrow and its power. Be careful, dear firefly, or else sorrow will dim your light.

A new storybook for Mice and Mystics! The storybook Heart of the Firefly directly follows the events of Sorrow and Remembrance and tells new adventures surrounding Prince Collin, Maginos, Tilda, Rex, Finger, and Lily. This expansion includes new miniatures, cards, and markers, as well as six new chapters that will present you with new challenges. Additionally, Neré joins the group of adventurers, a wild mystic whose knowledge of magic and alchemy will enrich the group's abilities.