The spirits from Beyond, from the roiling Chaos, the Abyss under the world. The chaotic spirits have no place in Cosmos, and when they come into contact with it, one or the other must change.
The spirits of Chaos do not seem purposefully malicious, many hardly seem to be sufficiently sentient to know a purpose. They are very destructive all the same, as their essences clash with those of Cosmos, and they are hated by the gods and spirits of Cosmos. Cosmos is as hostile a plane to chaotic spirits, as Chaos would be to cosmic creatures.
Chaos spirits known to the cultured races can be divided in three rough classes: Fetches, gremlins and reavers.
Fetches are chaos spirits that for some reason attempt to find a place in Cosmos. They roam Limbo and the Chaos beyond it, constantly seeking an angle allowing entry into the physical world.
Fetches take on the form of anything. They prefer intelligent, animate and (barring that) solid forms, but more often achieve insentient, inanimate and insubstantial forms.
The best way for a fetch to enter the material world is being brought back by a traveller through Limbo.
Fetches can also materialize temporarily, though usually in small, inanimate forms - depending on the fury of the aetheranemoi, fetches may be unable to materialize at all, or able to take animate forms. To keep existing, they must find a role in Cosmos, and the easiest way to do this is to get a native spirit, preferably a sentient one, to take an interest in it. If a higher spirit should turn its attention to the fetch, however, it will be cast out or destroyed.
Fetches, unlike two other classes of chaos spirits, are not malicious towards Cosmos in general. However, the only way most fetches can hope to find a place in Cosmos is by replacing something already there, and the best position of all would be that of an existing sentient creature...
Some say fetches are spirits found wanting by the gods, at the time of Creation or afterwards, condemned to hover in the outer darkness.
Fetches may have a "true" form, like reavers and gremlins, but when unable to occupy a specific cosmic form, they tend to drift through collages of cosmic entity traits, becoming such things as a half-bird, half leek, made out of rock at one point, morphing into a fiery icicle the next.
Gremlins have a constant influence on Cosmic life than the two other classes of chaos spirits. The aim of gremlin spirits is to bend and break the nature of cosmic things. Gremlins are things of disorder and entrophy, trying to keep fire from burning, rock from falling and life from living. They seem drawn to the activities of intelligent creatures and compelled to interfere with them. More than anything else, they favour interfering with complex devices and machinery, nesting in their aetherforms and pestering their native spirits.
Gremlins resemble reavers in form, but do not willingly interact directly with the Cosmos.
Reavers come from the deep primordeal Chaos. Cosmos is entirely alien to them, and they to it. Reavers are only seen or otherwise sensed when great magical forces are unleashed, typically around the turning of the year, or other times great, unwholesome magic is released, weakening the world's barriers.
Reavers often fail to register with one or more of the senses, they may be invisible, scentless, inaudible and intangible. When accessible to the senses, their countenance is most unpleasant.
Reaver bodies are asymmetrical and ill formed, and if some symmetry can be construed in their bodies, it is more often radial than lateral. To the extent that they resemble cosmic creatures, they are reminicent of deep sea fishes, crustacean, molluscs and starfish (the deeps of the oceans are closer to chaos than any other habitat). The sound of a reavers passage is discordant noise, a din of piping and screeching. The smell is unnatural, like that of an alchemist's workshop.
Reaver bodies seem unstable and shifting, and a reaver may morph into a totally different appearance in the wink of an eye, or in a slow metamorphosis taking hours.
Unlike fetches, who strive to insinuate themselves into Cosmos, reavers thrust violently into it. Whether they enter by purpose or accident, and whether any difference can be defined with such capricious creatures, is unknown.
Chaos natives in Cosmos have usually been brought here by powerful, unwholesome magic, though they may also come near the world before midwinter, when the world's barriers weaken.