House Morbidia was a Mechanicum-pledged Imperial Knight Household of ancient provenance that was founded at the very beginning of the Age of Strife, on holy Mars itself. House Morbidia started its existence as one of the very first Knight Households in service of the Omnissiah's priesthood and was one of the early rivals of the legendary Knights of Taranis and House Zavora. Yet where these Houses were to rise high in the esteem of the Martian priesthood, House Morbidia would fall in disgrace; sparkling an internecine war that would see it defeated and shackled to the will of the Fabricator-General. As an independent or even autonomous Imperial Knight Household, House Morbidia ceased to exist. In its place, as punishment for its crime the "Pact of Morbidia" was installed: a penal detachment -- never to be free or powerful again -- forced to act as the Fabricator-General's personal enforcers.
House Morbidia originated on Mars, in distant past when Humanity entered the Age of Strife. In early years it established itself as one of the early rivals of such famed Houses as House Zavora or even the Knights of Taranis for power and the favours of the Omnissiah’s priesthood. At first the House seem to have prospered, growing strong, as did its rivals. However, sometime during these dark and lost centuries, House Morbidia erred and committed crimes the truth of which have long been suppressed within the vast and ancient datacores of the Mars. The ruling Fabricator-General condemned their actions and pronounced them Excommunicate Traitoris. Thus began a series of bloody and bitter battles which would eventually bring the defiant scions of the House Morbidia to their knees in surrender, their power broken and their House in ruins. As the price for their survival, the Household was forced to accede to the crippling demands of servitude imposed by its new overlords, not least of all total submission to the office of the Fabricator-General "...until such times as the stars themselves burned cold..."
Since then, what remained of House Morbidia has formed a cadre of what could almost be described as slave-warriors, its scions condemned to live and die at the will of the Fabricator-General of Mars for the dim and forgotten sins of their forebears many generations before. Effectively, as an Imperial Knight Household, House Morbidia ceased to exist, but being bound to the Fabricator-General it was nevertheless equipped with Knight armours of the highest standard, having found a new purpose in the service of their master: that of punisher of those who would stray too far from the doctrines of Mars. On these occasions, the 'Pact of Morbidia' as it was henceforward known, often fought alongside the dreaded Legio Mortis, but was never permitted to become powerful again. On rare occasions, the ranks of the Pact were swelled by individual scions of other oath-bound Houses condemned by the Consistorial Courts of Mars to serve as part of the Pact for infractions of dogma and tech-heresy. However these sentences were seldom as severe as the one pronounced in eons past on House Morbidia, most Knights being sentenced to serve until their death or for a certain amount of years.
As the Horus Heresy dawned and the ominous Fabricator-General of Mars, Kelbor-Hal, sided with the Traitors, so too did the 'Pact of Morbidia'. Most of its number was scattered amongst the Traitor's Expeditionary Fleets across the galaxy, assigned to the Legio Mortis and Legio Magna detachments or dispatched to reinforce independent Dark Mechanicum battlegroups such as the Taghmata Satarael. Following the defeat of Warmaster Horus at the hands of the Emperor during the epic Battle of Terra, and the flight of the Traitor forces, it is assumed that House Morbidia fled with the Traitor Legions into the hellish realm known as the Eye of Terror, where they presumably still exist to this day.
- The Horus Heresy - Book Four: Conquest by Alan Bligh, pg. 99