Etherium is a magical alloy infused in the bodies of all Esper creatures. It has a look of intricate, polished filigree. The filigree replaces flesh rather than adding to it, giving Esper creatures an airy, gappy, almost insubstantial look. Etherium has been infused with aether.
The Ethersworn are a group of mages (of the human, vedalken, and even sphinx races) dedicated to infusing all life on Esper with etherium. Their quest is called the Noble Work, and they believe it to be the answer to the imperfect wildness of regular old organic bodies (which they call the Ignoble Flesh). But the Ethersworn weren't the first cultivators of etherium. In fact, they don't even have the recipe for making new etherium anymore—more and more, they have to reclaim the existing etherium from dead creatures and recycle it. As more creatures are infused with ever more etherium, the filigree structures have become attenuated, more intricate and delicate as etherium supplies dwindle. In essence, Esper is undergoing an etherium crisis.
So who has the recipe? A group of mages known as the Seekers of Carmot claim that they know the secret of etherium, and house it on their Filigree Texts in a secret chamber within their Esper compound. They say that etherium creation crucially involves a red stone known as "carmot," but so far, no new sources of carmot have been found on Esper.
An over-application of etherium can cause the subject to become etherium abominations called aether-liches
Decades ago, the sphinx Crucius devised the magical alloy etherium and began a grand project to infuse living things on Esper with it. Records show that he proposed the Noble Work as a means to overcome the frailties and limitations of the mortal flesh, but Crucius may have in fact perceived Esper's disconnection from two crucial elements, red and green mana. Indeed, the æther inside etherium, once spread across enough of the plane, may have been intended to enable a spell that would reunify Esper with the other planar fragments.