In Gustav Gloom and the Nightmare Vault, Fernie and Gustav find themselves battling another shadowy foe—The Shadow Eater. He is after one thing, The Nightmare Vault, which his master, the evil Lord Obsidian, will use to unleash the most terrifying and threatening shadows into the world. As Fernie and Gustav race to stop the Shadow Eater, Fernie learns about Gustav's mysterious past and just what happened to his missing parents.