Ophelia and the Garrison

The star had been designated "Ophelia" by the Mechanicum Explorator vessel that had first charted it. A yellow star similar to Sol, it was orbited by a dozen planets but only one was capable of supporting life. It was a nickel-iron cored world about the same size and mass as Terra, with six-tenths of its surface covered in water oceans and its landmasses heavily forested with thick equatorial jungles. Axial tilt, rotational and orbital period were all close to Mankind's homeworld and so it was inevitable the system would be colonized.
But first the native inhabitants had to be conquered. Ophelia IV* was populated - colonized would perhaps be overstating it - by xenos; a fierce race of warriors whom the Magi Xenologis surmised were a product of genetic engineering rather than natural selection. Certainly, there were no lifeforms they could have evolved from. They were intelligent, but their intelligence was solely devoted towards war - they had no civilization, no art or culture, no concept of relaxation or retirement. They had not built cities, only barracks, and showed no interest in the galaxy beyond their planet. Their weapons were simple but effective chemical-propellant firearms, produced in munitions factories they constructed instinctively.
They were tribal and different groups made constant war on each other, but they would capture rather than kill, trading and releasing captives freely - it was almost as if they were playing wargames rather than truly fighting. But when the Explorators' armies landed, they showed no such restraint and slaughtered the humans in hit-and-run tactics, practicing asymmetric defensive warfare. The natural cover afforded, not to mention familiarity with the terrain, meant the Explorator armies were halted and soon driven back with heavy casualties.
The Mechanicum, not given to flights of imagination, designated the xenos race "the Garrison" and made their report to the Imperial War Council. The lords of Terra agreed with the Explorators; the Ophelia system could not be ignored - not only was Ophelia IV ideal for colonization, but the system was strategically valuable and the other planets - while inhospitable - geologically rich. The Imperial Fist Legion - masters of siege warfare - were dispatched and the Emperor himself joined the assault, eager to seek answers to the questions posed by the presence of a race like the Garrison.
With First Captain Sigismund leading the assault a beachhead was established within hours. The Emperor and his court soon joined the Imperial Fists on the surface and war was made against the Garrison. With the might of Rogal Dorn's Legion, not to mention the strength of the Custodians of the Master of Mankind, the xenos were driven back towards the mountains which seemed to be the center, not of their civilization, but whatever it was they instinctively defended.
Though the fighting raged for several weeks the outcome of the campaign was never in doubt. Desperately, the Garrison massed for a last-ditch effort - not a final stand, but an all-or-nothing assault intended to kill the invaders' leaders. They gathered together - males, females and juveniles - at the head of a mountain valley and flung themselves at the Imperial forces. The Emperor himself lead the charge at the head of ten-thousand Imperial Fists and one hundred Custodians, slaughtering the xenos and destroying the Garrison as not just a fighting force but - for all intents and purposes - a race.
But while this final battle was being fought, other events were afoot. Seemingly out of nowhere, a group of Eldar - a Warlock and a bodyguard of Aspect Warriors - appeared behind the Imperial lines. The Warlock brought a stern warning and a dire prophecy; the mon-keigh had defeated the guardians the Old Ones had left on the planet - they must cease their offensive, abandon the world, and allow the guardians' numbers to recover.
The rearguard Space Marines who received the Eldar's message were indoctrinated to be suspicious of xenos, but something about the aliens' desperation gave them pause. Demanding the Warlock leave his bodyguard behind as hostages the Lieutenant and his command squad escorted the xenos to meet with his immediate superior.
First Captain Sigismund was not fighting in the valley - Rogal Dorn had wisely kept some of his forces in reserve in case the Garrison's assault was a trick, placing them under the command of his senior adjutant. But it seemed as if the Primarch's concern was unfounded - this was, truly, the Garrison's last-ditch attempt to drive the Imperials from Ophelia IV and in gathering their forces for this desperate gamble had all-but abandoned the mountains. Sigismund had pushed forward, all the way to the central and highest peak. There he had discovered what the Garrison were defending.
Something was hidden on the summit. Visually undetectible from any real distance, the very stone of the mountain had been softened with something akin to a melta weapon and then re-sculpted into a semblance of undressed stone as it cooled. The conclusion was inescapable; someone had hidden something on the planet and set the Garrison to guard it.
Scans revealed nothing - the stone could be penetrated, but something beyond it could not. Sigismund ordered the stone burned away with multi-meltas, and his Marines did just that, revealing a gleaming blue crystal sphere chased with unreadable golden runes. The tech-priests accompanying him seemed drawn to it, with an eagerness uncommmon among them. Interfacing with the strange runes, the seniormost said it was a door and it must be opened - a "great, ancient weapon" was inside, a weapon that would give victory who whoever possessed or controlled it. The other tech-priests eagerly agreed - the door must be opened. Sigismund, a straightforward warrior rather than a scientist or statesman, trusted the advice of the Mechanicum and ordered them to open it.
At that moment, the rearguard Space Marines arrived with the Warlock in tow. The xenos was aghast the mountain had been breached and "the Crystal Vault" exposed. Desperately, he implored the Marines and Tech-priests - Eldrad Ulthran the Farseer had cast the seerstones and read the runes; the mon-keigh did not understand what they tampered with. "The Seraph" must not be released - it was a great and terrible weapon that could not be allowed out into the galaxy.
The tech-priests challenged the Warlock; there was, indeed, a great and terrible weapon in there - but it was one the xenos wanted for themselves! It was Imperial blood, human blood, which had subjugated this world and discovered this weapon, and now the degenerate Eldar wanted to steal it? It must be put into the hands of the Emperor for him to use in the Great Crusade, not given to the filthy xenos.
The Warlock begged Sigismund, to understand - the Seraph couldn't be used, by anyone! It was uncontrollable, beyond the ken of even the Eldar, let alone primitive mon-keigh.
Sigismund listened to both sides, but it was clear he was swayed more by the arguments of the tech-priests - they were, after all, human rather than xenos. But it also made sense to him as a warrior; there was no sense in locking a weapon away and not using it - a weapon existed to be used. And the manifest destiny of mankind meant that it must be humans who used it - the temerity of the Eldar to say no human (not even the Emperor!) could use the weapon was the greatest insult. He dismissed the Warlock's entreaties as nothing more than xenos trickery - it was clear he wanted the weapon for himself. But no Imperial Fist, especially not the First Captain and the right-hand of Rogal Dorn himself, would fall for such lies. He would gain control of this "Seraph" and add it to the arsenal of the Legion for the greater glory of the Imperium and the Master of Mankind.
It is possible the Warlock knew Sigismund's mind before he did. Desperate, he sent out a psychic scream and charged forward, vainly hoping to kill the Marines and Tech-priests before they could make a terrible mistake. Sigismund cut him down like a child and ordered the tech-priests to redouble their efforts; the Crystal Vault must be opened and the weapon obtained before more xenos arrived to challenge them.
At the rear lines, the Aspect Warriors sensed their Warlock's call for aid and then his sudden death, and turned their weapons on those guarding them. Marines fell to shuriken, but the return bolter fire was brutal and precise and in a few moments Eldar bodies fell on Ophelian soil.
At the mountain, the tech-priests had interfaced with the crystal sphere, ancient runes shifting and tessellating as sequential encryption sequences were broken and locks opened. "Soon," the Tech-priests promised Sigismund. "Soon the vault will be opened and the Seraph will be free. Soon . . ."

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* Ophelia IV was, as its name suggests, the fourth planet from the Ophelia star. However, this was based on the initial astrography survey which failed to correctly designate a trio of dwarf planets which mutually orbit around their shared center of gravity. This trio - informally known as the Three Sisters - orbit the star between Ophelia IV / VII and the gas-giant Ophelia III. Periodically, the Three Sisters are caught by the gravity of Ophelia III and orbit that planet as satellites until they are flung free and back into their independent orbit by gravitic forces caused by planetary alignments within the Ophelia system. When the original astrography survey was made during the Great Crusade, the Three Sisters were caught and orbiting Ophelia III. This error was not corrected until after the Great Crusade, and dissemination of this information was hampered by the slowness of the Administratum. Accordingly, the name of the planet of the Synod Sanctorum is recorded as both Ophelia IV and Ophelia VII. In these records, the planet is named Ophelia IV prior to the Horus Heresy and Ophelia VII afterwards.