The Prospero Spireguards were once the the local Imperial Army/PDF's stationed to defend the planets of Prospero during the Great Crusade. However, after the Horus Heresy, the Spireguard, now corrupted by the forces of Chaos, would continue to fight under the Thousand Sons. They saw action in the siege of the Space Wolves homeworld of Fenris, this time led by Magnus the Red himself. Despite the corruption however, the Spireguards remains one of the most disciplined and cohesive Traitor Guard regiments in the entire Galaxy. This is thanks in no small part to the fact of Magnus's rule on Sortiarius who still remain sentimental over the loss of his original homeworld. This combined with the natural psychic brilliance of the Thousand Sons creates a competent military force that rivals many Imperial Guard regiments.
Overview & History
During the Compliance action on Ark Reach Secundus the Guard were noted to wear crimson greatcoats, heavy boots and silver shakoes lined with fur cut from the wings of the snow-shrikes, an indigenous bird used by the Avenians as weapons.
Officers were noted to wear crimson jackets decorated with brass buttons and gold frogging. They were also outfitted with silver helmets, curved sabres and laspistols.
A detachment of these men were onboard the warship Waning Moon under the command of Thousand Son Space Marine Mhotep during the early start of the Horus Heresy. They were set to board the mysterious vessel, the Furious Abyss, but were ultimately killed, when the Furious Abyss destroyed the Waning Moon. Only one rescue pod was able to leave, with only Mhotep aboard.
Modern Spireguard forces can vary a lot in it's appearance, though. While regiments drafted from the Sortiarius' population of Thrall Wizards generally keep the old style fueled by the aforementioned sentimental nostalgia of Magnus the Red, Thousand Sons maintain their grip on several planets outside the Eye and out of the view of the Imperium where they draft more expendable Spireguard regiments. Those "lite" Spireguards are pretty much your standard Imperial Guardsmen in both appearance and function, except with more respected psykers and no priests. Magnus famously mustered a million-numbered army of those lesser Spireguards for his first attack on Fenris in M32 and then left it all behind to cover his retreat.
They have been reborn and replenish by the time of the 13th Black Crusade.