The Aether is a coterminous plane of existence which can be accessed everywhere in the known universe. Like all coterminous planes, it underlines the entire universe, but laws of physics within it are different and can therefore be used to certain beneficial ends. Despite every point in the Aether being connected to a correspondent point in the material plane, the distances between those points are much, much shorter, greatly shortening travel times between worlds, which would otherwise take thousands if not millions of years. Civilization in the Galaxy have used this particular property of the Aether to their advantage since time immemorial, from the Titans to every modern day star-spanning empire.
The Aether is filled with a thick, misty substance also called aether. It has unique properties, conducing great amounts of energy at little risk and effort. Although it cannot be maintained in its natural state in the material plane, always dissipating into seemingly nothing, accessing the Aether to process aether into a powerful energy source was another staple of galactic technology. Despite its volatility, a few ancient cultures have been known to process aether into a stable form that maintains its cohesiveness in the material plane, known as astral diamonds. Only Titans, akashics and Yellowstone Progenitors are known to be able to produce astral diamonds, with only akashics still extant in the present day, although not sharing their methods. Although newly minted astral diamonds contain unimaginable amounts of energy, most known astral diamonds have already spent all of their energy and remain merely extremely rare and extremely valuable objects sometimes used as a high-end currency for particularly expensive transactions.
Although the Aether gives the galactic civilizations many blessings, it is not without dangers. It is also a spiritual plane where many spirits migrate towards upon death of their mortal bodies. Most spirits are not powerful, virtuous or vile enough to be of any threat to astral travelers, becoming merely specters wandering around with little recollection of their former lives. However, few spirits retain enough malice or magical power to manifest as powerful entities that can ambush and kill unwary travelers. When one's mortal body is slain in the Aether, the spirit remains imprisoned within it, making for a particularly nasty afterlife. The Aether is also allegedly the location of Heaven and Hell, located at the "top" and "bottom" of the plane, although no one has ever visited these places and lived to tell the tale (a few alleged heroes who got their loved ones back from Hell notwithstanding).
The Aether is a dimension shaped by belief, therefore there is no commonly agreed upon appearance. Although aether, when taken to the material plane, becomes a colorless, transparent mist in its natural state it appears to be a different color, and sometimes density, depending on the observer. More maritime oriented cultures see it as blue and watery, while more religious individuals, standing right next to the former, will report seeing a white-gold misty substance. Some report purple and green thick clouds or orange-red light. Because it is shaped by belief, a spirit powerful enough (living or dead) may exert their will to extract and isolate small pocket dimensions within the Aether. Small domains have been reported under command of powerful mages, but the most famous ones are divine domains shaped by gods and their worshipers. Within his domain, a god is even more powerful than on the material plane, being virtually omnipotent.
Because of its association with afterlife and gods, Aether is crucially important to many religions. Of particular note is Monadism which believes the Aether to be "the soul of the universe" and the ultimate expression of divinity. Monadism worships the Aether and sees merging back into it and losing individuality to that process as their ultimate reward. Once all the souls merge back into the Aether, it's supposed to wake up and remain perfect and timeless forever.
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