Ashandri and the Braythian Void

While Ashandri is a complete world of its own, it is important to understand its place in the Braythian Void and the connection to its surroundings. All that begins, of course, with the purple void: The infinite nothingness filled with thick purple clouds.
While only few inhabitants of Ashandri know this, the void has an important role in the continued existence of Ashandri. Any matter that leaves the world-island, from waterfalls to rocks that break off at the edge, slowly dissolves in the void. The newly formed purple clouds then float back to the underside of the world-island, transforming back into new material. In other words, the Braythian Void is the key factor of the world-islands not just crumbling away over time. If someone were to travel to Ashandri's bottom side - something that is entirely possible, if dangerous -, that person would see a thick purple cloud hovering under the rock like a pillow.
For some reason, the purple nebula steers clear of the chains that connect the worlds. In the rare occasion that a single streak of the mysterious element ever touches the metal of a chain, it gets sucked in like rain in a desert.

Entering the Braythian Void

There are many ways to enter the void: You can simply hop or climb off the edge of the world-island, or you could fly up into the sky until you pass the sky emerald - Ashandri's substitute for a sun. Some underground caves might even lead to the underside of the world, opening up into the thick purple cloud under the world-isle.
As soon as you come into contact with the void, something strange happens: Your basic life functions are shut down. Don't worry: You stay perfectly healthy. But your heartbeat stops, you cannot breathe anymore, even your blood stops pumping through your veins. Within the void, you need none of that to survive. You can eat and drink, but everything just stays in your belly unprocessed until you leave the void again. In theory, this property of the void could even be used for specific goals: Stopping the spread of a poison in the body, for example. Curiously, the void does not seem to affect aging. Resting in the void also works normally.
As the Braythian Void dissolves inorganic material falling off the world, it also affects equipment and - to a smaller extent - organic matter. For every thirty minutes here, there's a 10% chance for any one piece of equipment per person (chosen randomly) to simply dissolve into purple mist. There seems to be no difference between common materials and magic items. The only exception are true artifacts: Due to their inherent power, they are immune to the dissolving properties of the void.
Living creatures are, for some reason, protected against the instant dissolving effect. Instead, the purple void seems to slowly draw from their very life essence. For every eight hours spent in this environment, any living (or undead) creature has to succeed on a Constitution Saving Throw against DC 10. Failure results in 1d4 points of damage, which cannot be healed while within the void.
Vision and the other senses are not directly influenced by the Braythian Void, but the thick clouds block any vision and muffle any sound beyond thousand feet.

Magic and the Void

While most magic works normal in the Braythian Void, its life- and undeath-absorbing properties do have an effect on spells that revolve around these energies. Any healing or wounding effect that directly changes the hit points of the target - as from the spells cure wounds or inflict wounds - is halved. This includes magic potions, but natural regeneration like that from a troll still works normally.

Gravity

The void itself has no gravity; there is no up or down here. The world-islands have a very slight pull, though: Wherever you are in the void, the nearest world pulls you in by about a centimeter every hour (unless you actively move, of course).
The upper side of every Braythian world-isle is enveloped by an invisible half-dome, the highest point being where the sun-equivalent of that world hovers in the sky. As soon as a creature or an object passes into that dome, it is subject to normal gravity and instantly begins to fall downward (unless it can fly).
This in combination with the slight pull leads to the occasional inanimate object falling down - usually resulting in shooting stars, the object burning up before it ever hits the ground. Bigger or more resistent objects might come crashing down like a comet, though.

Movement in the void

Most of the purple clouds that fill the Braythian Void offer little to no resistance, so physical movement in the void is a bit complicated. Sometimes, a single streak or cloud becomes thicker than the rest. These condensed areas have properties similar to water. If they are big enough, it is possible to swim or fly through them. Smaller bits offer at least enough resistance to push yourself off of them at half your walking speed.
Once you are in motion in the void, you keep going until you either hit an object, or come upon another condensed area.
It is also possible to give yourself a push through spells like mage hand, or stop your movement with a wall of stone or similar spells. For that reason, having a spellcaster in your group while travelling the void is highly recommended.
Also, the thicker areas on the bottom side of each world-island always count as condensed areas.

Creatures in the Void

The Braythian Void is not only home to the world-islands and their connecting chains. A variety of creatures exist in the purple infinity, which can be put in four categories:
Travellers. Just like your group can journey through the void, there are travellers from other world-islands, too. Some might be exploring, others on important quests or even conquests. Some cultures have mastered the art of creating voidships - airship-like vessels that are not affected by the dissolving properties of the purple clouds, and that protect its crew and passengers from the life-draining effects.
Hollow Spirits. Even if you're protected from the life-draining, you can still die while in the void. Be it from wounds, magic or old age, if you die here, your body instantly dissolves into purple mist, and you turn into a disembodied spirit. If you succeed on a Wisdom Saving Throw against DC 10, you remember who you were - otherwise your memory is wiped entirely. In either case, your personality stays just the same.
In other words, the hollow spirits of the void can be harmful, helpful or entirely ignorant to others. They might even still have a task or an important quest to fulfill. They appear as translucent purple ghosts, vaguely resembling their original shape. The longer a hollow spirit exists, the more it transforms into a spherical shape. This process can take years or even centuries, but once finished, the sphere-spirits seem to gain a certain control over the purple clouds, gaining new powers from it. No hollow spirit can ever enter a world-island in the Braythian Void. If they come into contact with a world-chain, they get sucked in as if they were a stray cloud, instantly dying.
Resurrection magic can bring back a Hollow Spirit, but the individual has to succeed on another Wisdom Saving Throw against DC 10, or forget its entire time as a Hollow Spirit.
Void Wraiths. While hollow spirits are essentially the souls of living (or undead) beings, void wraiths seem to be something entirely different: They form out of the purple clouds themselves, usually taking on monstrous shapes of vastly varying size. These creatures - if they are actual creatures, and not just embodiments of the void - seem to be semi-sentient, instinct-driven, and infinitely hungry.
Void wraiths lurk mostly in the vicinity of world-islands, and consider any traveller as prey. They seem to be oblivious to danger, attacking even when they are completely overpowered. Their attacks not only drain the life force of their victims, but also nourishes them. When they have consumed a certain amount of energy, they evolve into a greater form, instantly healing in the process. More powerful wraiths can also consume smaller ones to heal their wounds.
What exactly the void wraiths are is hard to tell, but it is clear that they are directly linked to the Braythian Void, and can only exist outside of it for a short time (depending on the size of the wraith). They mostly avoid the world-chains and the world-isles for that reason. If they do not return to the void in time, they simply dissolve into nothingness.
Old Beasts. While some worlds like Ashandri are young, others have been transported into the Braythian Void eons ago. In theory, a world-isle in Braythe can exist for eternity. But some have a different fate, dissolving or losing any life essence that could nourish its inhabitants. While this usually spells a dark future for the locals, some powerful creatures manage to transform themselves. However they do it, these entities take on properties similar to world-islands: They roam the Braythian Void unaffected, and are even healed (or "reconstructed") by the purple clouds. Some of these creatures are so big (and benign enough) that they allow other beings to live upon their massive bodies, while others just hunt for prey, for power or for fun.