Either Ultramarines or Imperial Fists
Once, the Imperial Pillars upheld the edicts of the Emperor as exemplars of His grace. Now, their story stands as a grim reminder that the barest trace of corruption can be the downfall of virtue's mightiest paragons.
Created during one of the earlier Foundings, when mankind had yet to fully comprehend some aspects of Chaos it would discover in later millennia, the Imperial Pillars were a Successor chapter of either the Ultramarines or Imperial Fists, though centuries upon centuries of plots have left the Imperium with little in the way of evidence to confirm it one way or another. The only hints to either possibility come from their behavior in the story attributed to them. It is said that those veterans who founded the Imperial Pillars were great builders, and when they were gifted with the distant planet of Moirai for their homeworld, the second task they set before themselves after constructing a Chapter Monastery was to dig from the world's vast mountains a long system of sprawling caverns. Like the Ultramarines successor the Doom Eagles, the Imperial Pillars practiced the use of a grueling and lethal obstacle course as the first test set before its Aspirants, sending them into a maze of tight crevices and expansive caves to either perish or climb from the darkness a worthy Neophyte.
When they built this trial, however, their architects were careless, concerned only with setting the cleverest maze before their potential brothers, and dug too deep. They did not take the time to carefully search each new cavern they broke into, and thus failed to notice a lurker in the shadows: a single Horror, a Lesser Daemon of the Lord of Change. Had they been more vigilant, a single Astartes would have been more than sufficient for dealing with the wretched creature, but being so small allowed it that day to escape their sight. No longer bound in an exitless cavern beneath miles of solid rock, the daemon could have at last been free, but instead it stayed exactly where it had been for millennia already, and watched. Soon, the first Aspirants descended underground.
The trial before each Aspirant was one of endurance, resourcefulness, and willpower. On foot, the journey from entrance to the tunnel system's sole exit took days, climbing up and down slick rock in complete darkness, surviving on mosses, fungi, and other cave life. Slowly, however, anything they ate within the caves began to poison them, and if an Aspirant did not make their way quickly through the maze to receive the antidote from the Chapter Apothecaries, and even then only if they were found worthy, they would surely die, whether of poison or starvation. Those with the most wit and fortitude would learn to find the right paths and escape, but only once they had discarded any hope of surviving already. The unwise or just unlucky, however, fell deeper into the shadows, where the lone Horror would find them.
Alone in the dark, solid rock insulating them to any noise but what they made themselves, those who took the wrong paths would eventually lose not just hope, but the will to go on. Then, as they lay dying, succumbing to toxin thick in their blood or stomachs empty of food, the voice reached them. It gave them hope. It promised they would survive, for nothing in exchange except their word. Their binding word, which swore the lives that were saved to the owner of that voice. Then, the voice would whisper secrets of how to resist the poisons, and how to find the right way. And days later, the Aspirant would emerge, slender and filthy, but alive, and having known what it is to lose hope be declared worthy by the Chapter's Apothecaries.
With so many Aspirants surviving the trial, the ranks of the Imperial Pillars swelled within but a few short decades, much faster than practically any Chapter before, which the Pillars attributed to the quality of Moirai's potential recruits. Soon, they were first deployed in service of the Imperium, meting out justice to Xenos and Chaos alike. Their performance, however, while adequate, fell short of most other Chapters of the famed Space Marines. Their squads broke more often, their equipment's machine spirits less often cooperative. Most blamed simple bad luck; others, poor choices impossible to forsee the outcomes of; and a meager few went so far as daring to express concerns that perhaps the Imperial Pillars were casting their nets too widely when it came to accepting Aspirants. But before any scrutiny could be applied, the noose so carefully and gently tied, tightened.
A few mere centuries after their creation, and those Astartes who had survived the first trial thanks to their deal with the voice outnumbered their unsullied brothers. When the Chapter returned to Moirai to assemble in full after a vicious defeat at the hands of the Eldar, losing many of the loyal brothers, the owner of their contracts whispered now was the time their debts were repaid. Marking themselves with dye made from the poison moss they'd eaten long ago, the forsworn Imperial Pillars turned on their brothers and slew them to a man, executing their treachery either while their brothers lay unprepared, or en masse overcoming the veteran loyalists who'd founded their Chapter. When it was done, nigh on 600 Space Marines pledged themselves to Tzeentch through the Horror which had whispered into their ears. When Space Marines of another Chapter came to investigate the sudden silence of the Imperial Pillars' monastery, they found evidence of a Warp Portal torn open which had allowed the traitors to leave, and filled the emptied building with unspeakable abominations left behind as traps for the investigators among the corpses of the loyal dead.
Rewarded by their newfound god, Tzeentch mutated the tiny Horror into a Greater Daemon known now as the Slithering Tongue, and bid the remaining Imperial Pillars rename themselves. Understanding they were but tools in the plots of the Changer of Ways, he who manipulated the great machine of the material universe, they chose to name themselves his Dark Instruments. As one of the largest whole groups of Space Marines to embrace the worship of Chaos since the original Traitor Legions, the Dark Instruments were very successful in carving out a power block for themselves within Chaos' realm. They were, however, still prone to weakness, having been formed from lesser stock. For a time, they stood united, challenging even the most well-established Chaos Champions through their vast number, but in time, defeats saw them learn to change their ways. By the eve of M41, individual Dark Instruments have found their way into nearly every Chaos Warband of real significance which recruit openly in some capacity. They rank among the highest Sorcerers in the Black Legion and a few have made their way across the galaxy to join the Red Corsairs in various roles. Most deride them as the last of a failed Chapter which splintered when it entered the realm of Chaos, but a few suspect the Dark Instruments never truly disbanded. Merely split apart to find their own ways into positions crucial to some indiscernible plan, and now listen for some inaudible voice to give a signal. Listening, and waiting.