The Purging of the Moons of Ymgarl was a xenocidal Imperial military campaign carried out by the Salamanders Space Marine Chapter against the rare strain of Tyranids known as the Ymgarl Genestealers in 754-756.M41. With the coming of the Tyranid menace threatening the existence of the Imperium, it was realised with some shock that the insidious Genestealer threat, long a thorn in the side of the Imperium, was connected in some way to this newer, greater peril. By the edict of the High Lords of Terra, actions against suspected Genestealer infection were stepped up. As part of this xenocidal campaign, the Salamanders were given the onerous duty of purging the Genestealers for all time from the moons of Ymgarl. This was a realm where these foul creatures had always maintained a foothold and from which they had previously been thought to originate.
With the aid of the Magos Biologis of the Adeptus Mechanicus, the Salamanders constructed a detailed plan of attack, first rooting out their prey by using specially sanctioned alkem-weapons to burn away much of the moons' atmospheres and destroy the teeming jungles that covered them, thereby robbing their insidious enemies of sustenance and concealment before proceeding to cleanse the ancient tunnels that honeycombed the moons' surfaces.
The Salamanders advanced into the darkness in Mechanicus Termite boring craft with spider-stalker Servitors to scout their way forwards in the lightless underworld, methodically scouring the tunnels.
The origins of the Ymgarl Genestealers remain a complete mystery to Imperial savants, for they do not seem to have been a strain of bioform created by any of the Hive Fleets already known to have invaded Imperial space. It may be that they are the last survivors of a Tyranid reconnaissance of the galaxy before the arrival of Hive Fleet Behemoth. Tyranid Genestealers were first encountered on the moons of Ymgarl in approximately 500.M41, before the invasion by the Tyranid Hive Fleet Behemoth in 745.M41. They were the first Genestealers encountered by the forces of the Imperium of Man and were initially thought to be a new species of previously unknown intelligent xenos. It was later discovered that they were just one variant of the standard Genestealers deployed as a servant bioform of the Tyranid Hive Fleets.
The Ymgarl strain of Genestealer is a terrifying beast able to adapt to its foes and with physical power enough to outmatch even a Space Marine with ease. As waves of the inhumanly fast creatures came at the Salamanders, hissing from the darkness, battle was joined against a foe none but the Adeptus Astartes could hope to face and live. Deeper and deeper the Salamanders battled, encountering vast and strange blind wormlike mutations crashing through the tunnels, and appallingly powerful brood-lords whose taloned hands could shred a Terminator at a stroke. The losses among the Space Marines quickly mounted, but assaults from the nightmare Genestealers were despatched in storms of fire or crushed under the shattering blows of Thunder Hammers and the Salamanders did not falter despite desperate hours and terrible reversals. The campaign lasted for nearly two years of bloody struggle and death in the darkness before finally the Magos Biologis pronounced the ancient threat on Ymgarl ended. However, its dark legacy would live on in the shadows of chilled ship holds, dark Space Hulks and doomed worlds uncounted.
- Codex: Tyranids (5th Edition), pp. 15, 61
- Imperial Armour Volume Ten - The Badab War - Part Two, p. 78