Surface of the Shattered Moon
After the moon shattered, its pieces coalesced into a lifeless mass of rocks with the life-giving shards floating above them. The living require artificial means to visit the surface. The undead and the abominable wait there for those foolish enough to visit them.
Some of my best drinking buddies are Entulesse elves drowning their memories of the Surface. They tell me you have to stay alert on the edges of the shards. You never know when some haint or tentacled monster will climb up from the Surface to make a meal of ya!
During the Far War the moon was shattered by the invading forces from the Far Realm. In the years since Thaya's Moon has settled into a semblance of stability with the life-giving Shards of Thaya floating in a layer of breathable air, the driiathi-sphere, that begins a thousand feet above the new surface of the moon.
Although most of the activity on Thaya's Moon takes place on the Shards, the barren Surface makes up most of the moon's area. The vast expanse of jumbled rocks and settled dust hide swarms of undead and those fleeing elvish justice.
The Shadows of the Shards
The air close to the surface, the mairun-sphere, is too thin to support most life. Only the hardiest creatures can survive the Undershard for any length of time. The jumbled surface is composed of broken rock and dust that has just settled into place over the few decades since the Shattering.
Any life, such as the Undershard Snails, that exists on the Surface survives on the detritus that falls from the Shards above. Unlife, however, thrives in the Undershard. Vampires, ghosts, and other undead haunt the shadows, hidden from the burning gaze of the sun. These unnatural predators steal onto the Shards to prey on the living before descending back down into the darkness.
Far Realm aberrations dwell Undershard, but they go dormant in the thin air of the Surface. Any do require air to maintain a normal level of activity, so they tend to cling to the bottoms of the Shards, where they get the air they need while still hiding from the blinding sun.
Visiting the Undershard
Any living sentients traveling Undershard need to prepare against an agonizing death from suffocation or becoming a meal for a monstrous dweller from below. A typical setup includes a magical or magitech breathing apparatus, a pressurized suit, and a sun torch that gives off radiant energy that is deadly to the undead. Most important is a method to return to the surface, whether it's as simple as a pulley system or a method for magical flight.
Winged beings who fall into the mairun-sphere must exert themselves mightily, the equivalent of sprinting, to get back into the thicker air. Those that descend too deep are lost to the unforgiving Surface of the Shattered Moon.
Shards of Thaya
Floating islands on the Shattered Moon harbor the only life on that broken sphere.