Secundus Outbreak

The Secundus Outbreak was a xenocidal campaign conducted by the Crimson Sentinels Space Marine Chapter against a large, hidden cult of Genestealers upon the world of Dermanus Secundus. These Genestealer Cult was believed to be a remnant of the infamous Tyranid Hive Fleet Leviathan. Though the Crimson Sentinels put up a valiant defence of the capital hive city's population, they were vastly outnumbered by the chitinous horde of ravaging xenos. Issuing a desperate order to evacuate, the Crimson Sentinels sold their lives dearly in order to evacuate as many innocent civilians as they could, but in the end only a quarter of the population escaped the slaughter. With no other recourse, the only way to wipe out the genestealers was through a massive orbital bombardment, which utterly decimated the hive city, leaving a vast smoking crater of ash and rubble.

History
As the seven-year long Armada Rebellion was coming to a close in 659.M34, a full-on assault, led by High Sentinel Andreas himself, was conducted upon the last Hive City under the control of rebellious Lord of the Hive World - Armada Incarus VII. The city was besieged by a large Crimson Sentinels force, supported by heavy armour, as they pushed the Lord of Secundus' armies back to the spire of the city and soon Andreas came face-to-face with Armada. He would aim his bolt pistol towards the coward, ready to pull the trigger for his transgressions against his brothers and the thousands of lives he has taken from the surrounding planets as a whole. However, he never got the chance to, as he was interrupted by the sounds of screaming and panic outside.

Draco Neatherstorm would inform Andreas of a sudden Genestealer outburst in the Hive City, as it was quickly filled to the brim with these vile xenos. Unbeknownst to the Crimson Sentinels or the government of the Hive World, a Tyranid Genestealer cult originating from Hive Fleet Leviathan, had been brewing in the Hive City for decades, growing in size and constant growth until it was ready to burst.

The Crimson Sentinels forces were surrounded within the city as the Genestealers crawled up from the lower levels of the city. They were greatly outnumbered by the massive horde of xenos. Andreas had little options, so he was forced to take the most dire of actions. He issued an evacuation of all citizens of the city, sacrificing his brothers for the safety of others to hold off the xenos. The evacuation resulted in only a quarter of the population of the city to survive the outbreak, along with a single company and the honor guard of the Crimson Sentinels making it out alive. The city was quickly decimated by a massive orbital bombardment by the Crimson Sentinels vessel, The Spear of Sanguinius, turning the city into rubble and wiping it from the planet's surface. All that was left of the once magnificent Hive City was a crater and a reminder of the Rebellion. Armada Incarus VII was presumed dead after the bombardment.