The Compliance of Caldera was a joint pacification of the Aeldari-controlled world designated One-Five-Four-Four and also known as Ibsen by the Salamanders, Death Guard and Iron Hands Legiones Astartes during the latter years of the Great Crusade in the late 30th Millennium. After the end of the campaign the newly-purged world was renamed Caldera.
This was an unusual campaign of the Great Crusade that involved bringing the world of 154-4, known as Ibsen by its inhabitants, into Imperial Compliance. This world had been discovered by the Iron Hands 154th Expeditionary Fleet led by Primarch Ferrus Manus. Upon closer inspection, the Iron Hands realised that this world was most unusual, as it was a rare example of Aeldari / human coexistence.
Upon further inspection, it was discovered that these humans were in fact descendants of the Salamanders' homeworld of Nocturne who had been liberated from Drukhari captivity by their Craftworld kin. The Salamanders Legion, lead by the Primarch Vulkan, were joined by the Death Guard and the Iron Hands Legions, both led by their own respective Primarchs. The world was undeveloped, and largely inhospitable to human life, but possessed valuable mineral deposits. However, the Imperial forces faced stiff resistance for control of the planet from the Aeldari, who had placed a garrison upon Ibsen.
The Aeldari forces—including Seers, Warlocks and other combat psykers -- could not have expected to defeat one Legion of Astartes, let alone three. The mystery deepened when the semi-feral, primitive human tribes inhabiting the planet seemed more sympathetic to the Aeldari, or at the very least, not welcoming to their human liberators.
The Salamanders and other Imperials defeated the Aeldari relatively easily as expected. After the conquest, the Salamanders learned that the Aeldari had been defending a network of menhirs which served as psychic nodes and that fed into a giant arch, where the final and most brutal confrontation between the Astartes and the xenos had taken place. The arch itself had been located thanks to a mysterious Remembrancer attached to the Salamanders, with whom Vulkan had had an unsettling conversation before the battle.
After the slaughter ended, Vulkan saw a Remembrancer loitering by the arch and then suddenly disappearing. Following him, Vulkan and his elite Pyre Guard descended into the chambers beneath it, through a portal at its base. There they found a crude warding ceremony taking place, conducted by the primitive human tribal priests, who were ready to sacrifice an ancient Drukhari witch.
Finally, Vulkan realised the truth: the arch was in reality a Webway portal (though none of the Imperials, including Vulkan, knew this at the time); Vulkan had seen a "gate" like this before, in his Nocturnean youth, when Drukhari repeatedly utilised such a portal to raid and pillage the planet. The Aeldari had taken control of the portal by defeating their dark cousins, and in the process had freed the indigenous human population of Ibsen from the horrors of Drukhari raids; the Aeldari had been seen by the native humans as liberators.
Terrified by the defeat of their liberators at the hands of the Imperium, the natives had sought to sacrifice their Drukhari captive to ward off the inevitable return of her race; the mysterious "Remembrancer" who was nowhere to be seen had actually been the Emperor in disguise, who must have had a great interest in the particulars of this mission since he had ordered so many resources committed to it.
In the meantime, the Iron Hands Legion was charged with bringing the desert regions of the planet into Imperial Compliance, but the campaign did not bode well for the Imperial forces as they were hindered by the inhospitable terrain and the fluid battle tactics of the xenos that took a savage toll on Ferrus Manus' forces, placing him well behind schedule in purging the world.
Unbeknownst to Ferrus Manus, two Aeldari discussed his fate. One of them decided to intervene, to ensure that Ferrus Manus walked along the more positive of the two possible paths leading to his future. The other Aeldari warned against such interference, reminding his companion that "Stone cannot bend, it can only break."
The Iron Hands pushed on into the deep desert and arrived at a suspicious valley. For reasons that he could not explain, the Primarch felt that this area was important somehow, but he was loath to investigate it further without a Scout force. Unwilling to wait for Imeprial Army support, the Primarch ordered the Iron Hands to descend into the valley. As soon as the bulk of their forces reached the floor of the valley, a mysterious darkness engulfed them, followed by severe inclement weather.
Aeldari warriors lay in wait and quickly ambushed the disoriented Legionary warriors in close-quarter battle. They also employed a mysterious weapon that overrode the Iron Hands' bionics, blinding those with augmetic eyes and overriding and controlling bionic hands that turned their weapons upon their fellow Battle-Brothers or themselves. The Iron Hands assault force was only saved by the timely arrival of Bion Henricos' rearguard force, who took out the Aeldari Warlocks responsible for the malefic psychic attack. The Aeldari were forced to retreat, leaving the Iron Hands to recover. But things soon took a drastic turn when it was noticed that Ferrus Manus was missing.
Ferrus found himself within a cavern where he was assaulted by a barrage of prescient visions and perceptions of death -- of his Legion, his Astartes and even himself. He also saw metaphorical representations of his fellow Primarch brothers and was perplexed by feelings of betrayal that were engendered when he gazed upon a statue of his closest brother Fulgrim. During this ordeal, Ferrus felt as if he was being pursued by a malevolent serpentine entity. Irritated by such sorcerous parlour tricks, the Primarch pressed onward and explored the caverns further.
Within the cavernous depths, Ferrus Manus continued onwards until he was suddenly attacked by the serpentine creature that had been following him. The massive silver wyrm proved to be a vicious and deadly foe, and nearly slew the Primarch several times. During the ensuing battle, the Primarch was distracted by a sense of déjà vu and a vague sense of prophetic vision concerning the words Angel Exterminatus, which appeared in his mind unbidden.
Concocting a counterattack against the massive creature, Ferrus was inspired by the memory of his brother Vulkan and managed to wound the serpent, forcing it to retreat. Elsewhere, the two Aeldari responsible for sharing these visions with Ferrus Manus lamented their allowance of an aspect of Chaos to encroach into their demesne when they kidnapped the Primarch.
Ferrus Manus continued on his strange psychic journey, eventually reaching a bizarrely constructed, ancient throne with a rotting corpse sitting upon it. Coiled around it was the great serpentine wyrm that he had encountered earlier. The Iron Hands Primarch came to believe that this must be the spirit of Asirnoth, the dread wyrm of Medusa, that he had slain long ago. Wrestling with the massive creature, he was shocked by its sudden transformation as the wyrm shapeshifted into the all-too familiar form of Fulgrim, which informed the Primarch that he is not Asirnoth.
Angrily, Ferrus slew the corrupted half-Primarch/serpentine beast, before tearing the corpse-king from its throne, opening a concealed doorway behind it. The Primarch then confronted his Aeldari kidnappers and demanded his release, brushing aside their claims that they were merely trying to forewarn him against a likely future. Frightened for their lives in the face of the enraged human demigod, the Aeldari granted the Primarch his leave.
Eventually Ferrus reemerged on the planet's surface, and rejoined his warriors in assaulting the Aeldari forces. When asked about his recent whereabouts, the Primarch refused to answer questions about his recent experiences, claiming that nothing of importance had occurred. In response to Vox hails from his brother Vulkan, Ferrus then led his Iron Hands into the jungle regions of the planet.
In another place entirely, the two Aeldari rued their failure, but took solace in knowing that even if the Primarch known as The Gorgon should fall, there was still another that would suit their purposes.
Eventually, Vulkan ordered Ibsen and its irredeemably xenos-corrupted population cleansed by flame. He renamed the scoured, Nocturne-like Dead World that remained Caldera. The world was now ready to receive new human colonists and to be exploited for the benefit of the Imperium.
- The Primarchs (Anthology) edited by Christian Dunn, "Feat of Iron" by Nick Kyme
- Promethean Sun (Novella) by Nick Kyme