The Taintclouds are a poisonous layer of gas which separates the Sky from the

The Sfida, lying low above the Taintclouds

polluted Earth below. The clouds were created by the Ancient Wizards of Duhr to absorb the poison from the earth after the War of The Gods and return the earth to its natural lush and fertile state.

This natural barrier also serves to prevent those living in the sky from knowing what's beneath; it's believed that any who go beneath the clouds never return and that no creature can survive in them. For those reasons, the Taintclouds are sometimes considered to be the boundary between this this life and the next in the lore of the Sky's population.

Sky taints clouds

The Taintclouds being sucked into the dimension from which Cor Hydrae emerged after the defeat of Malpercio.