Warforged

Steam-belching automatons of brass, suits of gleaming armour animated by vengeful spirits, human-like simulacrums of steel - all are examples of Warforged, one of the newest "species" in the world.
Warforged are creatures made of inanimate matter given life by sorcerous means. They harbour as much, if not more diversity than their creatures, their only limitations being the construction of their shell and the spirit (or spirits) used to give animus to their lifeless forms. Animantic magic, a close relative of necromancy, is their origin, and for that reason, the creation of Warforged is mysterious and controversial in equal measure. Furthermore, the creation of a Warforged requires a spirit: an animal, a willing volunteer, or a ghost, dragged screaming from the underworld...
The Constructs of Tznchundel:
The oldest known warforged came from the defunct dwarven realm of Tznchundel, in the Irontooth mountains near the Barrowmere Valley. Originally, the constructs were built as a way to honour beloved ancestors. When an exceptionally esteemed elder was on the brink of death, the Archons of Tznchundel, the city's magocratic council, would make an offer to transplant the soul of the soon-to-be-deceased into a core of Soulstone, which would then be transplanted into a large steam and brass automaton. These automatons - referred to usually as Centurions - would aid their descendants in tunneling and warfare and took an honoured place in the society of Tznchundel. 
As the dwarven realm came under attack from the depths of the world, more and more warforged were needed on the front lines. And so the Archons made the decision to begin transplanting the souls of the dead - involuntarily! The King of Tznchundel, horrified by this, vetoed their decision. In response, the Archons killed him, and made him the first of their Warforged thralls. Over the following centuries, Tznchundel became ruled by a depraved death-cult of Archons where the souls of ancestors were wrenched screaming into cold, unfeeling shells, thrown against the Skaven and illithid as fodder for a brutal war. Yet it was not enough, and the city fell despite its mighty artifice. 
The warforged of Tznchundel, now exceedingly few in number, are physically made of brass, and belch steam and hydraulic fluid from their creaky joints. They have great diversity in form, with some of them able to roll into mobile spheres, and others taking the shape of spiders. Often powered by multiple souls, nearly all have descended into utter madness, their spirits tortured by decades or centuries of being trapped in cold steel, entombed in dark passageways.