The Cleansing of Hive Thetus was a miltary campaign against a Tyranid Genestealer Cult discovered by the Imperial Inquisition to exist upon the Hive World of Nevaria II and ultimately successfully purged from existence by the Black Templars Chapter of Space Marines. The Black Templars were called upon to aid in the extermination of a small-but-dangerous heretical cult on the Hive World of Nevaria II. This menace was investigated by an agent of the Inquisition's Ordo Hereticus working within the overpopulated levels of Hive Thetus, a Nevarian hive city. Soon, this agent discovered the towering spyres of Thetus held a more sinister secret than anyone had at first suspected. A powerful and malevolent influence was behind the spread of the cult's influence.
Further investigation, along with a very costly covert strike by a Kill-team of the Ordo Xenos' Deathwatch Space Marines, uncovered a powerful Tyranid Genestealer Broodlord at work. The creature had somehow managed to infiltrate and infect the minds of a large portion of Hive Thetus' population without anyone knowing. Years of breeding the Genestealers' foul xenos DNA into the human population of the hive city, as was their standard modus operandi when preparing a world for the eventual consumption of all of its biomass by a Tyranid Hive Fleet, had produced multiple hybrid Genestealers who were an abomination to the purity of the human form and a grave threat to the life of every still-pure man, woman and child on Nevaria II. It became obvious that Hive Thetus was becoming a staging ground for a dreaded Tyranid Hive Fleet invasion. The hive city had to be purged if the rest of the world was to have even an opportunity to avoid an Inquisitorial Exterminatus order. The Black Templars stepped up to the challenge of returning the Emperor's Light to Hive Thetus with their usual righteous zeal.
The Broodlord and its foul kin managed to anchor themselves deep within the hive's blackened depths. The removal of this taint would take some considerable time and effort on the Black Templars' part. The labyrinth of tunnels, accessways, ducts, and forgotten settlements in Thetus' underhive proved to be a deadly and unique battlefield. It was here that the Black Templars fought terrifying battles in cramped corridors against the Tyranid menace. Slowly and at great cost, each sector of the hive city was cleared from the bottom up. Hive Thetus, and the entire world of Nevaria II with it, was saved from the purging fire of the Exterminatus only due to the singular efforts of the Black Templars.
- Codex: Black Templars (4th Edition)
- White Dwarf 312 (UK), "Righteous Victories" by Graham McNeill