The Deadlands

"The Deadlands" is the general term for the world below the Fractured Skies; a seemingly endless wasteland of undeath and ruin. Hidden from the view by a dense, near impenetrable fog, the Deadlands can only be observed by actually descending to the surface, a death sentence for most Sky sailors.
The air of the Deadlands is poisonous to all living beings, as it infused with the ever lingering energy of Nul. Creatures who spend any length of time in the fog slowly start to lose their ability to sustain themselves, before eventually succumbing to total organ failure. Those who perish in the fog then rise again, as Nul animated dead, driven by a insatiable hunger for living flesh.

Geography

The Deadlands are covered in the ruins of an impossibly massive city that seems to cover the entirety of the planet's surface. Great gashes exist on the surface where the island's rose, but these do not necessarally correspond to where the islands eventually ended up in the Skies. 
Some natural features are undeveloped (such as oceans and mountain ranges), and a few areas of the Deadlands below are covered in dense unnatural forests that somehow thrive despite a lack of light.

Ecosystem

In addition to millions (if not billions) of animated dead, the surface is also plagued by great Nul Beasts, who hunt the dead, the living, and each other. Horrible mutants that blend aspects of different creatures into nightmarish amalgams of teeth, claw, and flesh, their origin and purpose (should they even have one) is unknown. 
Rumours also persist of Nul Source Dragons existing on the surface, dark mirrors of the various elemental avatars that guard the Source Crystals which keep islands floating. Though, if these monstrosities do exist, there has been as of yet no survivors who can creditably provide an account as to what they look like or are capable of.

History

The Deadlands was once the home of the Precursor Race, before their world fell to some unknown apocalypse. Whatever happened to them somehow also resulted in the rising of the Sky islands and the creation of the Fractured Skies we know today. Nearly every culture has their own beliefs regarding the end of the old world, but none can agree on a consistent narrative.

"I know this is going to sound crazy, because it is, but I heard her singing. In my head. As she ripped apart Halm's throat, she was singing a song about her favourite doll. The clothes she was wearing... I think she must have been a kid when their world ended. I don't think she stopped growing since then, Titians, she must have been four meters tall!

I think she let me go. I think she was satisfied, for a moment, with Halm. But ever since I saw her, I haven't been able to get her song out of my head. And lately, I've been having trouble keeping anything but meat down."