The Sidon Protocols designate a complex web of oaths of fealty and servitude, contractual obligations and pledges of support that form the basis of the ties between the Forge Worlds of the Adeptus Mechanicus and Knight Worlds across all of the Imperium. In essence, the Sidon Protocols are a bilateral treaty benefitting both the Forge World and its attached Knight Worlds. However, not all Imperial Knight Households are subject to these protocols, as it has been the policy of the Imperium for millennia to tie the rediscovered Household's loyalties to itself rather than to the fractious Priesthood of Mars. As such, the Sidon Protocols only apply to independent Knight Houses that chose to align themselves with the Mechanicum rather than directly with the Imperium, and that have not been fully absorbed by their patron Forge World.
Oathbound Vassal Households of the Mechanicum
At the core of every Knight Household is a panoply of arcane artefacts that defy the understanding of all but the most learned Adepts of the Mechanicum. From the mind-impulse thrones from which the Knight armours are guided, to the Ion Shields that ward the enemy's wrath from their imposing hulls, without these mysterious technologies the Noble scions of the Knight Households would be utterly powerless against the larger foes they might face; and so it is far from surprising that many Knight Households have strong ties with the Mechanicum. Few Forge Worlds would spurn the loyalty of such a mighty force, representing as it does both a devastating weapon to wield as well as a potent symbol of the divine Machine Spirit, and many a bargain has been brokered between Forge and House.
Unlike their more independent brethren, those Households sworn to the service of the Mechanicum are a strange breed, centuries of service to the creed of the Magi who dominate their patron Forge having warped their traditions and appearance. Often these Houses stride to battle in machines far different from the ancient Paladin-type armours, bearing strange and terrible weapons not seen upon the battlefield since the Dark Age of Technology. Some are even rumoured to have been inducted into the lesser mysteries of the Mechanicum's secret arts, studded with esoteric Augmetics, or even sealed eternally within the tomb of their Knight armour as illustrated by the bonds between House Hermetika and their patron Forge World of Mezoa. When the Taghmata of the Mechanicum muster for war, they are joined by these secretive warriors and the thunder of their war-horns heralds the victory to come.
Those Knight Households that remained in contact with one of the far-flung Forge Worlds of the Martian Cult during the Age of Strife have most often fallen into a symbiotic relationship with their patron, receiving technological and spiritual guidance from the Magos, while themselves acted as a potent shield for the Forge World. Many Forge Worlds might have faded from history during the Age of Strife were it not for the tireless efforts of the Knight Households bound to their service. Such a joining of the cold logic of the Mechanicum and the impassioned zeal of the more typical Knight Houses led to many widely varying oaths. Most common among these oaths is the form that has come to be known as the Sidon Protocols, a complex web of obligations and duties that bind both House and Forge in a dangerous accord, whose tangled precepts can easily lead to the loss of any semblance of independence for a Knight Household that fails to uphold the protocols.
Amongst the more well known rights provided by the Sidon Protocols is the right of the Household to bear the symbol of the Opus Machina, or Cog Mechanicum, the cog wheel-set skull, as part of their panoply of arms. This is both a sign that they carry with them the authority of Mars and the Cult Mechanicus, as well as a mark of their duty as part of the Taghmata of their patron. This also serves to set them apart from their Imperial-aligned brethren, though both took upon themselves the title of Questoris and joined the Expeditionary Fleets of the Great Crusade, the mark of the Opus Machina showed that ultimately they owed allegiance to the Mechanicum and the ambitions of their patron Forge World and not to the Emperor of Mankind. This subtle distinction led to some strife amongst the armies of the Great Crusade, and led to those Households pledged to the Mechanicum gaining a sometimes misleading reputation for untrustworthiness amongst Imperial Commanders. Matters were to become worse during the dark years of the Age of the Horus Heresy as Mechanicum-bound Households, true to their oaths, sided with their patron Forge Worlds for the Traitor Warmaster Horus and against their Emperor to become part of the Dark Mechanicum.
The Sidon Protocols also allow the Forge World to assign an Archmagos Prelate to the homeworld of the Household. Ostensibly, the Prelate is somewhat akin to the Executor Fetial found amongst the Legions of the Collegia Titanica, a diplomat between House and Forge who functions as an advisor and observer within the Household, seeing both to the well-being of its Knights and the training of its Sacristans, but in some cases stands as a quiet tyrant behind the throne of the Household's master. The Prelate also functions as a link between the scions of the Households and the amorphous structure of the Forge World's Taghmata. To the Household he is a representative of the Forge World, but within the constantly fluctuating network of prestige and alliances that is the Mechanicum Taghmata system, he is the representative of the Household. Considering the military power of a Knight Household, such a position brings great influence to the Archmagos Prelate, and many a Household has become inadvertently enmeshed in the intricate plots of a Forge World's Synod of Magi as its members vie for the position.
The majority of the remaining common precepts within the Sidon Protocols deal with the interplay of House and Forge; with the numbers and types of armours to be supplied and the conditions and duration of a Household's service under the banner of the Taghmata. It is this co-dependency, the exchange of irreplaceable machinery of war for trained and seasoned warriors whose skills are superior to those of brute robotic Automata, that binds the two disparate organisations together. Such a close relationship has led to many Households developing curious hybrid cultures, often retaining much of the independent nature of their neo-feudal origins but alongside often bizarre interpretations of their patron's brand of the Cult of the Omnissiah, much to the consternation of the Archmagos Prelate.
Gilded Fetters of Blood and Iron
Only a few Knight Households maintained contact with the domains of the Mechanicum during the Age of Strife. Undoubtedly many of those that did not have long since perished, overwhelmed by the terrors of Old Night, or have lost the last of the Knight armours that marked them out, devolving into simple Feral Worlds that maintain only the neo-feudal traditions of their origins. Those few that did survive the trials of the Age of Strife, prevailing alone and unaided, were much sought-after prizes for the expanding armies of Mankind.
The Expeditionary Fleets of the Great Crusade went to great lengths to secure such worlds, prizing the skills of a Household's warriors and the power of their Knight armours. However it has long been the Imperium's policy to secure the loyalty of these rediscovered Households directly to the Emperor and for Imperial factotums to represent the Household in all dealings with the Mechanicum. For, while a common citizen may see the Imperium of Mankind as one realm, it is more of a loose alliance of competing human political powers that ever seek to outdo each other. Yet, despite the efforts of the Emperor's servants, some few Knight Households escaped the Great Crusade's attention only to be chanced upon by Mechanicum Explorator vessels alone or ceded to the control of a Forge World in exchange for some portion of its industrial output or the service of its assembled Taghmata.
Such rediscovered Knight Worlds, often sorely in need of new armours and lacking any long association with their new patron Forge, found themselves caught in something of a devil's bargain. The dire need for technological expertise and replacement armours serve as a simple leash for the coldly logical Archmagi of a Forge World to wield in order to seal a Knight Household in to its services, and one that is applied with chilling efficiency. Where in some cases the Sidon Protocols serve to bind together House and Forge in amicable alliance, in such instances the complex terms of debt and duty were manipulated to strangle the political independence of the Household. This was itself a pale reflection of the worst excesses of the Mechanicum in this regard during the anarchy and desperation of the Age of Strife, when Households bound to such a fate could expect little courtesy from their new masters, and inevitably functioned as the vanguard of any military operation conducted by their patron Forge's Taghmata, serving ultimately as an expendable shock assault force. Nor did they expect to receive the resources the Mechanicum granted to those Households able to leverage either long-standing alliances, or an established force of arms, and were purposefully kept short of fully functional armours and often only supplied with older patterns and less powerful configurations of arms. The least fortunate of such Households are rumoured to have been forcibly "enhanced" by the shunned Adepts of the Lachrimallus on particularly wayward Forge Worlds, leaving them literal slaves to the will of their Archmagos Prelate; unsleeping, fearless warriors whose only desire is the final escape of death. Although not quite so extreme, the fall of House Atrax under the sway of the Cyclothrathe Mechanicum serves as a dark exemplar of what could become of such Houses. It is believed that during the horrors of the Horus Heresy and the Great Scouring, this practice, suppressed during the Great Crusade, saw a hidden revival, though if this was the case on the Loyalist side, such Thrall-Houses have long since been destroyed or hidden from the sight of the Emperor's servants.
Blade of the Omnissiah
Regardless of the means by which a Forge World exerts control over those Knight Households in its sworn service, its seasoned warriors and their arcane combat walker mounts are precious assets and quickly incorporated into the forces available to that Forge World. The Archmagos Prelate assigned to the House will normally represent them within the Forge World's Synod, a gathering that is neither meant for those un-indoctrinated in the higher mysteries of the Cult Mechanicus, nor easily understandable to the un-augmented, and decide on their deployment should the time come that they are summoned to war.
Most commonly, a Mechanicum-oathed Knight House bound by the Sidon Protocols will be required to assign a cadre of Knights to any significant mustering of the Forge World's Taghmata, serving for the duration of the campaign under the overall command of the Archmagos leading the Taghmata, but serving under their own officers. Within the Taghmata of the Mechanicum, Knights serve a vital role as shock assault troops, valued for their independent initiative as much for their raw power. In massed battles, their first role is as line breakers, breaching an enemy's ranks to enable the cybernetic Skitarii Legions of the Cult Mechanicus to sweep forward and engage the weakened foe, or serving as escorts to the gigantic war machines of the Ordinatus Locum and Ordo Reductor. There are few Forge Worlds willing to commit to wholesale battle without the support of at least a single allied Knight House, and many of the most well-established Forge Worlds can call on several should the need arise.
Knights and the Titan Legions
It is also common practice among those Forge Worlds that maintain a Titan Legion to assign allied Knight detachments to Titan maniples as skirmishers and scout elements. These Knights are expected to screen the larger Titans from infantry formations and strike tanks, and many Magi view such an assignment in support of the God-machines as a distinct honour. It is common practice amongst those Knights granted the privilege of fighting alongside Titans to apply elements of the Titan Legion's heraldry to their own armour. In such actions during the Great Crusade, the most terrifying and powerful xenos races were purged from existence in deadly combat, and few Knights survived long in the shadow of the mighty war machines they guarded in such battles. The Princeps of the Legio Titanicus were proud to honour the sacrifice of the scions that marched to war with them in this manner, and afforded their allied Knight Houses great respect, and those who returned to their homeworld on both sides often retained elements commemorating their fellowship in their personal heraldry as a mark of honour.
Those Households that maintained some measure of political autonomy, whether through strength of arms or steadfast loyalty, often retained the right to sanction independent military operations or to join the ranks of the Ordo Questoris -- the Household forces that were independently detached into the service of the Expeditionary Fleets of the Great Crusade's armies. Such Houses were driven to seek glory among the far-flung stars of the galaxy, far from their patron Forge Taghmata, or even engaged in their own conquests; as loyal to the ideals of human unity and the Emperor as ever they were to their Mechanicum patrons. The riches and reputation brought from such independent martial endeavours served in some case to aid them in gaining greater autonomy from their Mechanicum patrons, which is why many Archmagi were loathe to grant such rights as part of the Sidon Protocols.
- The Horus Heresy - Book Four: Conquest (Forge World Series) by Alan Bligh, pp. 96-98