No one except the Warforged know where they came from or how their species came to be. Four hundred years ago, at the peak of the Larassan Empire, they arrived on the shores of the continent on a great ship of metal. At first the Empire thought it was about to be invaded; instead they were surprised when the constructs sent out emissaries. The terms they offered were simple. They would serve for a time as mercenaries for the Larassan Empire, and provide the Empire with the process for creating new Warforged. In exchange they would be recognized by the Empire as free beings, and once their contract with the Empire was up, they would be free to work for anyone they chose. The Empire, so arrogant that it believed itself to be invincible, and seeing the potential of being able to produce an army of such warriors, agreed. What the Warforged didn’t tell them was how difficult and expensive the process was, nor the fact that there was no way to control a produced Warforged. But by the time the Empire discovered how they’d been tricked, the deal was made. And despite the Empire’s vengeful efforts to wipe out the Warforged by throwing them into the worst, most suicidal engagements, the Warforged thrived and reached the end of their contract. Since then, the Warforged have served as the best mercenaries on the continent, working for whoever will give them the most interesting war and the most interesting techniques.
In regards to appearance, the Warforged vary widely from individual to individual. The ones that have been created in the years since the Warforged appearance on the continent generally have a resemblance to humans (think Warforged as they usually look). Those who have been around since before then, or those created by such ancients tend to be much less humanoid. Some appear almost like sword wielding mantises, merciless insectoid monsters of metals unknown on this continent. Others have a distinctly raven-like shape, without wings but still much faster than a thing crafted of steel should ever be. And the third group the strangest of all; interlocking, and ever-rotating wheels covered in eyes and many-jointed limbs clutching an assortment of ancient weaponry. These are the oldest Warforged, and the most deadly (Assume they’re all level 15-20 Fighters, or similar martial class).
The Warforged culture such as it is revolves entirely around the evolution and perfection of their warrior skills. In their mercenary work, the Warforged accept training in lieu of much of much coin. If they can be taught how to wield a new weapon, or a new tactic, or even given the chance to fight a new foe, then they will drastically reduce the costs of their service. Warforged do not hesitate to work with each or against one another. Interestingly, while they will fight one another, no one has ever seen another Warforged kill another. Wound, yes, but never kill. The constructs are silent when questioned on the matter, as they are on most subjects unrelated to war.