Ymgarl, located somewhere in the galactic north-west, is most notable for its moons being the supposed origin of Genestealers, in particular the Ymgarl Genestealer strain, before the Imperium had knowledge of the Tyranids as a species.
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The Genestealers were thought to spread off the moons to other parts of the Galaxy by infiltrating passing cargo ships and derelict Space Hulks. It was not until the Battle for Macragge that Genestealers were observed to be used as shock troops by the Tyranids, and further investigation of the alien bodies after the battle confirmed that all Genestealers were part of the Tyranid race.
In response to this revelation, the Salamanders undertook a xenocidal campaign to purge the moons of Ymgarl and Inquisitors increased their alertness levels for possible Genestealer infestations. However the Ymgarl Genestealer strain was not entirely eradicated by the Salamanders and many still lurk in hibernation in the dark corners of the galaxy.