The Assault of the Tempest Galleries is the rare battle of the ancient Unification Wars that united Ancient Terra under the control of the Emperor of Mankind in the late 30th Millennium that is still recorded in Imperial archives. The Caucasus Wastes was a techno-barbarian state located in the Caucasus Peninsula of the ancient European continent on Old Earth. Its despotic ruler, known as the Ethnarch of the Caucasus Wastes, was defeated by the forces of the Emperor and imprisoned in the maximum security Imperial prison in the Himalazian (Himalayan) Mountains known as Khangba Marwu, The Vault. The primary Imperial force responsible for this outcome was the early XVIII Legion of Space Marines, later known to history as the Salamanders.
Although it is entirely likely that the transhuman warriors of the XVIII Legion saw action before their assault on the Caucasus Wastes, the records of these engagements being neither deliberately lost, deleted or sealed at the highest level, the first open battle honour recorded for the then as yet unmatched XVIII Legion was the assault on the Tempest Galleries during the overthrow of the Ethnarch of the Caucasus Wastes on Terra. It would also prove to be one of the most famous early campaigns for the Legion. Even though it occurred long before the XVIII's reuniting with their Primarch Vulkan on Nocturne, this conflict set its seal upon the nature of the Legion and had a profound influence that remains to this day.
The conquest of the Caucasus Wastes was one of the last great battles of the Unification Wars that would see at last the Emperor as the undisputed master of Terra, but victory did not prove easy. The forces of six entire proto-Legions of the Legiones Astartes were mustered for the final assault on the Caucasus Wastes, along with the massed forces of the Legio Custodes, with the Emperor Himself to lead them, and countless other tributary armies, mechanised battlaions and warrior bands beneath the raptor-and-lightning banner of Unification. The first thrust upon which all would depend to carry the assault was entrusted to the nascent XVIII Legion. The full force of what could still only be characterised as a proto-Legion, some 20,000 Astartes strong at this juncture, was committed to what was widely believed to be a suicide mission to destroy the vast geo-thermal furnaces which provided power for the Ethnarchy's impregnable defences. These were housed far below even the subterranean strongholds, and known of only through ancient legend, in a hundred-kilometre-long string of vast, artificially created caverns known as the Tempest Galleries for the incessant storms of flame and electromagnetic force which bled from the ancient and little-understood relic machineries.
Their specially-modified Power Armour, camouflaged in striated patterns of sulphur yellow and sable black, the XVIII saluted their Emperor and calmly mounted the untested Martian war engines of the Mechanicum and pierced the dark earth and rock at the edge of the wastes, disappearing into a deadly and unknown world. Soon all signals were lost, and not even the power of the Imperial psykers could penetrate the turbulent depths to maintain contact with the assault forces. After nearly a solar month underground, great tremors and earthquakes shook the region. In their wake, the power-webs that had for so long shielded the Caucasus Wastes failed and the full force of the Emperor's wrath fell upon them to their utter destruction. Of the fate of the XVIII Legion nothing was known until the closing days of the campaign when, far from the wastes, a long-dormant volcano near Klostzatz shattered and one of the Termite machines hauled itself from the rubble carrying just over a thousand survivors from the XVIII. Their fantastic tale of bravery and unbelievable sacrifice became known throughout the Legions. The Emperor Himself gathered the survivors together and awarded them the Laurel of Victory (to the dissatisfaction of some commanders) to be forever theirs, and their first battle honour, inscribed with the motto "In Fire and Darkness Tested." This was to prove prophetic, as the standard of sacrifice and victory against the odds would prove a punitive taskmaster for the XVIII Legion in the early years of the Great Crusade.
- The Horus Heresy - Book Two: Massacre (Imperial Armour), pp. 19, 41, 114-135, 193, 246-253