From the publisher's website
Deep beneath the ground, in those regions where the echoes of mankind’s feet are seldom heard, where even firelight is a bizarre intruder from a long-forgotten place, the foes of mankind breed and multiply. It is a realm of dark chaos – for these creatures of the echoing deeps count each other as rivals and enemies. It is a place without peace, a place without virtues, a place where every countenance of evil is to be found in sluggish torpor, stirring restlessly in dreams of cruelty and hatred. These dark and deadly provinces are known as the Under Realms.
Most named regions in the Under Realms are quite distinct from each other, whether by geological formation or by the types of inhabitants that have established themselves in strategically important areas. Characters who enter this area expecting to find drowic elves, deep gnomes, and other "normal" features of the Under Realms will swiftly discover the error of their thinking. The Vaults of the Sunless Sea, located to the north of the Cyclopean Deeps, might fit that description, but the Cyclopean Deeps are a different and decidedly more unpleasant area into which the denizens of the Vaults seldom venture.
This series of adventures forms a mini-campaign in the depths of the earth, suitable for a party of 4+ characters with an average level of 10-12, or 8+ characters with an average level around 7.