A sulvyn is less a creature than a thing the open sky does now and then. High above any ground a person could stand on, where the air layer presses closest to the world, the ambient field of Sulus sometimes gathers itself into a coherent shape and moves as though it meant to. For a few breaths it is a being. Then it is only wind again.
It carries nothing that a living thing carries. No soul to climb to the Astral when it ends, no shadow to sink to Malstaris, no spirit to hold a name. A sulvyn is Sulus and nothing else, the element wearing a shape for as long as the shape will hold. By the honest reckoning it is not alive, and it is barely animate even while it lasts.
What keeps them rare is the same thing that keeps the air layer hard to work at all. Sulus almost never thickens into a seam that breaks the surface; there is no rich fissure for a sulvyn to condense out of and stand thick beside. It forms instead from the thin field that lies over everything, and only high up, where that field runs least thin. The thin field cannot hold the form long. A sulvyn lasts moments where a fire-construct lasts centuries, and then it comes undone.
Off the larboard rail, two hundred fathoms up and falling behind us: a turning column of clear air with grit and a torn gull-feather caught in it, holding the rough outline of something with a head and a reaching arm. It paced the hull for the length of a held breath, curious the way a dog is curious, and then the outline lost its edges all at once and there was only the wind and the feather coming down.
While it holds, a sulvyn behaves like the feeling that feeds its element. It is restless and it is curious and it will not be pinned. It follows things that move through the high air, a climbing bird, a kite cut loose, a warm updraft, drawn along for a while and then gone after the next thing. It does not settle anywhere, because settling is the one thing air will not do. This is also why no one has ever bound one or kept one. The temperament that makes a sulvyn is the temperament that scatters it.
There is no killing a sulvyn, because nothing in it was ever alive to kill. Disrupt the shape and it simply stops being a shape. The air that was briefly a sulvyn goes back to being air, and the coherence drains back toward Sulus faster than a guttered flame goes home to its seam, because the sulvyn barely held together to begin with. It leaves no body. It leaves no remnant and no haunt, and there is no name in it to be forgotten. A sulvyn that comes apart leaves less behind than the feather it was chasing.