From the announcement:
Cities across the globe are reeling from the effects of Shudde M’ell struggling in his prison beneath the Heart of Africa. The seals that had trapped him beneath the ancient city of G’harne are weakening. The earth’s crust cracks and heaves, and the very fabric of reality warps and splits. As we’ve shown in our two previews, cities that were once safe havens can now be wiped entirely off the map by disasters that arise amid Shudde M’ell’s dreams. Even though this devastation is a serious setback, unique challenges await amidst the ruins that could yield great insight and valuable assets for your investigators.
As the great cthonion creature stirs, the investigators must meet this incredible threat with new strategies. Gain an array of advantages by arming your desperate team with a folio of Agency Secrets, or attack the creatures head-on by unleashing some powerful arcane forces of your own with the Wave of Destruction task.
Both of these Unique Assets are new to this expansion, and are just a sampling of some of the new Asset, Condition, and Spell cards you’ll find in Cities in Ruin. Take advantage of every last scrap of resources that you can gather, for the Cataclysm from Below is waking, and the investigators must race to stop him.