A Knight is a small combat walker of the Adeptus Mechanicus that is one-man version of a Titan and is smaller than even the smallest class of true Titan, the Warhound-class Scout Titan used by the Titan Legions. Knights normally deploy in squadrons comprised of several of these war machines. The most common type of a Knight is about nine metres tall, with its head serving as the cockpit. Knights are the smallest and weakest of Mechanicus walkers, but they still are formidable and fearsome machines, all armed with an array of weapons beyond the power of even the strongest of Space Marines to bear.
During the Age of Strife, when so much of the galaxy was made off-limits to interstellar travel because of the massive Warp Storms that were making the Immaterium impassable as the Chaos God Slaanesh gestated within the Empyrean, the Cult Mechanicus of Mars sent out many starships that made use of sublight drives to explore the galaxy in search of Standard Template Construct (STC) technologies on lost human colony worlds. In the course of their travels, these slow-moving exploratory expeditions discovered large clusters of human-settled star systems that had once belonged to the ancient interstellar human confederacy that had existed during the Dark Age of Technology. But these worlds had regressed during the Age of Strife to become Feudal Worlds possessed of a preindustrial, feudal culture dominated by castes of warrior aristocrats. These nobles, called the Knights, welcomed the Tech-priests to their planets, and saw the Mechanicus as saviours who could return to them the knowledge and technologies of their ancestors. The Priests of Mars settled among these star systems, which they named the Knight Worlds, and preferred to place their outposts on planets that possessed a good base of strategic minerals, which could be used as the foundation for the creation of advanced industries. The Mechanicus' representatives established full diplomatic and commercial relations with the Knights and were also very interested in the ruins of advanced cities that existed on many of the Knight Worlds, where a knowledgeable scavenger could sometimes recover very useful pieces of lost technology. The Knights provided Mechanicum settlements with the manpower and military force required to drive off incursions by Orks or Eldar Exodites and the Mechanicum in return provided the Knight Worlds with their advanced technology and the skills needed to rebuild their lost civilisations.
In time, the planets colonised by the Mechanicum were transformed into powerful bastions of industry and advanced technology that are known today as Forge Worlds. The Tech-priests and the Knights became mutually dependent, and with the Mechanicum cut off from Mars during the Age of Strife, each of the Forge Worlds created its own stellar empire consisting of the Forge World and its surrounding Knight Worlds.The Knights absorbed all that the Tech-priests had to teach and in time their planets became technologically sophisticated industrial societies once more, though they retained many cultural characteristics of their feudal pasts. Since the Tech-priests provided their technology only to the nobility that ruled each Knight World, these lords used their monopoly over the new knowledge to maintain the essentially feudal character of their societies even as industry returned and science advanced once more. Despite the limitations of Warp travel during this period, many of the Forge Worlds managed to retain limited contact with the other Cult Mechanicus outposts across the galaxy and with Mars itself using the sublight starships that had brought them out into the galaxy in the first place, though these journeys could take many Terran centuries.
The same pattern as described above would be replicated by the Mechanicum during the Great Crusade and perhaps even afterwards. Forward supply stations and other settlements would grow to become fully fledged Forge Worlds, and the planets surrounding them would be colonised by the inhabitants of the Knight Worlds.
The most important technology that the Tech-priests provided to their Knight World clients were the great robotic war machines that themselves soon became known as Knights as they were viewed as the ultimate embodiment of the mounted warrior. These war engines were much smaller than true Titans and could be piloted by only a single man, but they were well-suited to the highly-mobile, heavy cavalry-centric warfare preferred by the aristocracy of the Knight Worlds. In the past these warrior nobles had clashed with each other in honourable combat upon their horses -- now they could still face each other with honour, but did so from the cockpits of their war engines, their minds cybernetically neuro-linked to the machine's systems through Mind Impulse Unit implants in their spines and heads.
When the Age of Strife finally ended in the 30th Millennium and the Emperor of Mankind's alliance with the Mechanicum in the Treaty of Mars created the Imperium of Man, the Knights were incorporated into the armed forces of the Mechanicum and were even deployed for a time to aid the Imperium in the conquests of the Great Crusade after ca. 800.M30.
But the Knights were never as powerful or effective on the battlefield as their massive Titan counterparts, and over time Imperial commanders began to make use of the Knights far more infrequently. Eventually, most of the Knight Houses were ordered to remain on their homeworlds and the Forge Worlds they owed fealty to in order to serve as reserve forces for the Mechanicum. Knight squadrons were called up to serve alongside the Titan Legions only in certain situations where their greater mobility was a real asset. During the Horus Heresy in the early 31st Millennium, when the fighting spread to the Mechanicum as it broke into Loyalist and Traitor factions just like the rest of the Imperium's military forces, the Knights played a more prominent role in the battles between the Mechanicum and its Dark Mechanicus counterpart. A number of Knight Houses came to the worship of the Ruinous Powers during the Horus Heresy, and supported the Warmaster Horus and the Dark Mechanicus in their war against the False Emperor. When the Heresy ended, these Traitor Houses presumably fled into the Eye of Terror alongside the rest of the Forces of Chaos, and remain active to this day, now twisted and changed by the mutating power of the Warp. In modern times, the Houses that remained loyal to the Imperium still serve and are occasionally called to fight alongside the Titan Legion of their parent Forge World, but in these times they act only as second-line forces.
All Knight Worlds in the Imperium of Man owe fealty to the Magi of the Adeptus Mechanicus, and in particular to their "capital" Forge World, bound by oaths sworn thousands of standard years ago. In times of war, every Knight World sends thousands of men and machines, in addition to the Knight Titans, to fight for the Mechanicus wherever they are requested. In addition to serving the Forge Worlds, the Knights will support campaigns of the Imperial Guard at the behest of their Mechanicus masters. The Knight Worlds supply the great Forge Worlds with food and raw materials, and are most often the main source of these resources. Most of the Knight Worlds have several large landmasses covered with plains or vast forests. Through these wildernesses wander grazing herds of Megasaurs, great dinosaur-like beasts that either evolved naturally or were transplanted to the planet during the Dark Age of Technology. Carnosaur predators pursue and hunt the herds, and the Knights in their armoured suits are required to fend them off. The Knights keep a constant vigil in order to keep not only the natural predators, but also pirates and raiders from the Eldar Exodite clans at bay. The Servitor-run strip mines often become assaulted by those who covet the gold, silver and uranium extracted from there.
For an entire year the Knights defend their domains until great ships arrive from their capital Forge World, bringing new Knight Suits, weapons, tools and mining machinery. They leave with their holds packed with ores and Megasaur meat. At times Warp Storms are severe enough to delay Mechanicus fleets from reaching their destination for months, leaving the Knight Worlds teetering on the brink of anarchy.
The basic tactical and sociopolitical unit employed by the Knight Worlds is a House. A Knight House is an aristocratic noble family that owes fealty to the Adeptus Mechanicus, providing soldiers, the resources of their worlds and their own martial skills in exchange for the technological expertise that the Tech-priests of Mars offer. A Knight House is usually headed by a senior member of this family, aided by other elders of the House. Some Knight Worlds have many different families inhabiting them, whilst others are dominated by a single one. On planets with more than one House, the rivalry for the favour of the capital Forge World is high and sometimes breaks into hostility. Despite their personal differences, however, when the Forge World gives the call to arms, all Knights fight side by side against the enemies of the Mechanicus and the Imperium.
In times of war, a House's formation -- Knights and the men-at-arms who provide tactical support -- that goes on Crusade with Imperial forces is known as a Household. Knights are further separated into detachments according to their type for greater tactical flexibility. A Household is under overall command of a Seneschal -- usually, but not necessarily, a Knight Baron. A Household is further divided into detachments, which by principle can only contain Knights of a single type.
Knights are not commonly now seen on the battlefields of the late 41st Millenium. Most often they only enter combat in defence of Mechanicus domains or of their own worlds. They sometimes provide tactical support to a Titan Legion in a Crusade, or to units of the Imperial Guard if their help is ordered by the Lords of their House's allied Forge World. On the other hand, when a force of Imperial Guardsmen acts in support of a Knight Household, the Knight Seneschal technically can take command of the Guardsmen, acting as their company commander for the duration of combat operations.