The Prophets of Mercury was once a Loyalist Chapter of Space Marines of unknown origin and Founding. It was lost after entering the Eye of Terror in 321.M37 as part of the Imperial Abyssal Crusade. It is unknown what it's Battle-Brothers endured during their sojourn into the Eye, though they were declared Martyr Extremis by the Imperial Inquisition, considered to have been wiped out by the Forces of Chaos.
Following the Ecclesiarchal Purges of 321.M37, a dozen star systems were engulfed by Warp Storm Dionys, its echoes rippling along the spiral arms of the galaxy as it raged through the Empyrean. Records of mutation and Chaos Cultist activity quadrupled overnight. Worse yet, it was not only the citizens who were affected by the sudden influx of Chaos. Many of the Space Marine Chapters with homeworlds affected by the Warp Storm found that the secret imperfections in their gene-seed were writ large upon their new recruits, giving rise to a wave of disturbing manifestations both physical and psychological. The Prophets of Mercury was one such Loyalist Chapter of Astartes that had been affected by these Warp Storms.
When the Ecclesiarchy heard of this sinister turn of events, Saint Basillius the Elder demanded that all those Chapters of the Adeptus Astartes whose homeworlds had been touched by the Warp Storm be rendered unto his judgement. Such was the elder's influence with the High Lords of Terra that within a standard year this had come to pass. After a series of stringent tests and prognostications, hundreds of Chapters were deemed unaffected by the Warp Storm. No less than thirty were found wanting. The Judged, as these fallen Chapters came to be known, volunteered for a redemptive Penitent Crusade. The most militant of their number demanded the right to purify their tainted flesh in the fires of battle, to make a noble end from tragic misfortune. To the surprise of his closest advisors, Saint Basillius agreed to this proposal. He saw it fitting to send the accused into the Eye of Terror, taking the fight for the Imperium's future to the Daemon Worlds inhabited by the Traitor Marines.
A representative from each of the doomed Chapters held an emergency Council of Dismay to discuss the proposed Crusade. After scant hours of debate, they acquiesced to Basillius' demands, for they believed that martyrdom was preferable to an existence of suspicion and doubt. The last few days of 321.M37 saw a solemn procession of Strike Cruisers and Battle Barges pass through the Cadian Gate into the Eye of Terror, relay systems dormant and heraldic colours obscured by black mag-plates. One by one, the Chapters of The Judged disappeared into the iridescent dust nebulae that surrounded the Eye. As the massive Space Marine flotilla entered the Eye of Terror, they were set upon by a massive Chaos Warfleet. The resultant battle was so fierce that the ships of The Judged were forced to retreat and were scattered to the furthest corners of the Eye.
Chapter after Chapter fell to the perils of the Eye, for the Abyssal Crusade had entered a hell from which very few emerged unchanged. The true account of what occurred within the Eye to the Prophets of Mercury is not known, but most of the tales of the Chapters of The Judged ended in tragedy and sorrow. The current whereabouts of the Prophets of Mercury are unknown and the Imperial Inquisition has declared them Martyr Extremis -- slaughtered to the last Astartes in battle against the Archenemy.
The Prophets of Mercury's Chapter colours are not listed in current Imperial records.
The Prophet of Mercury's Chapter badge is not listed in current Imperial records.
- Codex: Chaos Space Marines (6th Edition), pg. 19