Dungeon World: Exodus
Leviathan is the last thing the world will ever see.
It appears from nowhere, heralded by a noise which ripples across the land. The creature resembles a whale but with strange, changing colours shimmering as the world’s light changes. It flies with no known direction or purpose. Living on it are patchwork monsters, insectoid demons who drop from the back of Leviathan and scour whole towns on unspeakable quests.
Its’ followers have found items long hidden which are strange to the world. It always feeds and it cannot be stopped.