In first edition Promethean, the Unfleshed are an optional addition presented in Saturnine Night. They are not a Promethean Lineage, but they are related to the Created; for the Unfleshed, the Fire has given life to a machine, not the corpses used to build a Promethean.
The Unfleshed have no associated element or humour. They select their Disquiet and Torment from those of the Promethean Lineages, though both need not come from the same Lineage. However, all of them share the same Wasteland, as the Pyros interacts with their inorganic bodies in similar ways.
The earliest recorded Unfleshed is Aeolipilus, a creature of clockwork and steam power, created by Dr. Montgomery Kelly in Victorian London. Whether he is the first Unfleshed, or whether there were others before him, is unknown.
What is known is that since then, the Divine Fire has come repeatedly to demiurges working on mechanical life, spurring them to create new Unfleshed, each of which is effectively the equivalent of a Promethean Progenitor, with the potential to found their own Lineage.
The majority of Unfleshed are created by human demiurges; Unfleshed created by their own kind are few, given what is required to make them.
There are certain identified requirements for an Unfleshed demiurge; invariably someone in science or engineering, possessed of great learning, intellect, and creativity, attempting to recreate or refine the mechanisms of human biology in technology. They need not have set out to do so - someone working on new robotic systems is also a candidate for demiurge, as the Divine Fire imbues the demiurge with the drive of creation.
Regardless of what the original creation looked like, the Divine Fire reshapes it upon completion into something human-like - still mechanical, but in the image of a human being.
An Unfleshed wishing to create another of their kind must build, or have built for them, a body similar to their own. They must imbue a piece of their own technology with Pyros, place it in the body, and then touch the body, making sure they are in contact with the Pyros-infused parts. Finally, they must invest the new creation with Azoth.
For the most part, the Unfleshed share the innate properties of the Created, with certain differences.
Though the Unfleshed are mechanical, they still need to eat, fuelling the Divine Fire within them. They are capable of eating non-organic material that can be chewed and swallowed, but this can serve to distance them from emulation of humanity, if their emulation is good enough. They are immune to natural poisons and diseases, and can use Pyros to ignore supernatural ones.
The Divine Fire provides a human disguise for the Unfleshed as it does for Prometheans. However, some aspect of their mortal form will always come off as artificial or unnatural in some way. Their true mechanical bodies will be revealed with the expenditure of Pyros.
While Pandorans will feast on the Unfleshed as they would Prometheans, given the choice they will target Prometheans over Unfleshed. However, Gremlins, the Mockeries of the Unfleshed, will devour Prometheans as they would the Unfleshed, but will prefer Unfleshed over Prometheans.
The Unfleshed are just as capable of pursuing Refinements and becoming human as Prometheans, but given their greater distance from humanity, the drive to pursue the Great Work can be easier to resist or put off.
In addition, since nothing in them has ever been human, they must try and understand the human body from the outside, learning about the body's benefits and costs, how it develops and how it works.
In second edition, the Unfleshed are a full Promethean Lineage, presented in the second edition corebook. They suffer from an excess of the oil humour, making them obedient, volatile, and hard to control.
The Unfleshed's origins are lost in the mists of time, with claims being made for the golem of Prague, Talos, or even Galatea as the first of their number. Some Unfleshed say they have no single origin, having been around since the first artificial copy of the human form. Some go so far as to claim the Unfleshed were the very first Prometheans, created from the earliest representations of humanity.
The only requirements for a new Unfleshed's body are that it must be artificial, it must follow an essentially human body plan - no extra arms, legs or heads - and it must have a function, must have been built to do something. Unfleshed have been created from statues, idols, mannequins, robots and chatbots.
All of the Unfleshed began as functional tools created to serve specific purposes, and all of them excel at whatever it is they were built to do. However, while they are tools, they aren't slaves - they want themselves and their work to be treated with respect, and being abused, insulted or dismissed causes them to lash out, struggling to control emotions they're still not used to.
Ideally, the Unfleshed's Pilgrimage sees them turning from the logic, perfection, and clear sight of their purpose they possess as artificial beings, to the emotion, subjectivity and fallibility that come with humanity, becoming tools that have learned how to use themselves.