During their long history, the Guardians of the Covenant have fought a great many wars and crusades. Though most have been lost in the sands of time, forgotten save for a tattered banner of withered scroll somewhere deep in the Spire, there are some battles which have shaped the history of the Chapter and the people of Mortikah as we know it.
The Castellax Campaign was a joint operation by the Ordo Xenos and the Guardians of the Covenant. The campaign was centred around the forge world of Castellax which was beset from all sides by a vast ork invasion fleet. Castellax had been an important forge world during the Great Crusade, producing all manner of war gear of the Legiones Astartes. As of the forty-first millennium, the world was heavy with war relics that could never be replaced if lost.
The Inquisition answered a desperate plea by the Adeptus Mechanicus that a vast ork fleet was approaching. The xenos were bent on looting the the planet and moving on to the nearby heavily populated sub-sector. If the orks succeeded at Castellax, the Imperium would be thrown into another bloody war of attrition.
Warp travel is fickle, and it took a good part of four months to journey from Mortikah VII to Castellax. During that period, the vile orks gained a secure foothold on the south pole of the planet. Using it as a staging ground to invade the rest of the planet by looting enough precious war machines to build their own ramshackle abominations. The Mechanicus-aligned knightly House Rajax together with the Legio Silentium and the local Taghmata Omnissiah barely held back the green tide.
At the beginning of the fifth month of the journey, the Black Ships of the Inquisition together with several battle barges and strike cruisers of the Guardians of the Covenant, translated back to realspace near the Castellax system. The Space Marines deployed with drop pods and thunderhawks and even a few storm eagles made the descent, bringing the Emperor's Fury upon the vile orks. The battle was costly, as the orks already looted enough to make gargants and other heavy weaponry. In the end, the forge world of Castellax was liberated. The Adeptus Mechanicus, as a token of gratitude gifted three precious relic dreadnought chasses, which were kept in stasis for millennia, to the Chapter. And thus was the bond between both factions was further strengthened.
The Kheres Crusade, or informally named the Crusade of Angels due to the Space Marine Chapters involved, was an operation between the Blood Angels and the Guardians of the Covenant. The former were exacting revenge on the vile hrud. The fighting was fierce and both forces fought to a standstill. Ever more hrud poured from the warp and the strike force consisting of the Blood Angels third company simply did not have the numbers and resources to wage a bloody war of attrition. What the Blood Angels were doing in a remote corner of the Imperium battling the hrud, no one but they themselves knew.
Though far from Imperial space, their pleas for assistance in battling the hrud were not misplaced. The Guardians of the Covenant Fourth Company answered the call in force, relieving the embattled Blood Angels and driving back the vile warp-spawned xenos. In gratitude of this timely intervention, the Blood Angels gifted a dozen ornate angelus-pattern jumppacks, resplendent with ceramite wings. Normally worn by the Chapter's elite, the Sanguinary Guard, the Grand Master of the Guardians of the Covenant fashioned them for his own armour and the remaining others for his Honour Guard. They can still be seen wearing the jumppacks to this day.
The undertaking which eventually became known as Lucian's Folly started as a crusade into the Halo Zones to claim several uncharted sectors for the Imperium. By request of the High Lords of Terra, fully half of the Chapter was committed to the crusade. This force, led by Grand Master Lucian crusaded forth to bring the Emperor's light in the remote darkness of the Halo Zones. To Imperial scholars, this event is known as the Armeneus Crusade, but to the people and brothers of Mortikah VII, this tragic even will be forever etched into public consciousness as Lucian's Folly.
The undertaking begin with great applause and fanfare, as a great many Mortikans gladly joined the Astra Militarum in order to partake in the crusade. The departure of soldiers for the crusade was of a magnitude so immense, that the place where the many ships lifted off eventually grew into a bristling spaceport, named in honour of the lost Grand Master.
Where the Armeneus Crusade began full of wonders, glory and Imperial prowess, it ended in utter tragedy. Five months into the campaign, all astrotelephatical communication was lost after a warp surge seemed to brutally end the lives of every telepathic choir tuned in to the location of the crusade. Attempts to re-establish communication has proved futile, all traces of the crusade, which had numbers well into the hundred thousands as well as half a Chapter worth of Astartes, went cold. No contact was ever made again. The Imperium was set back by this loss, but the Chapter was sent reeling by the loss of five hundred battle-brothers, including the Grand Master and many veteran members of the Chapter. Never again would half the Chapter be committed again to one single campaign.