The Hellgrammite were a mysterious and powerful xenos empire based out of the Laanah Rifts. Their true name is unknown and is only a title given by Imperial soldiers, for they showed no interest in communication with humans and were extremely hostile. Described as foul anthropoid creatures with decayed physical forms, they possessed sinister occult-infused weaponry and rode into battle on the backs of bio-mechanical monsters and abominations capable of ripping through heavy armor with ease.
During the Bellrath Crusade into the Lanaath Region in early to mid M38, the Imperium attempted to subdue the Hellgrammite but found it extremely difficult. Their throneworld of Wormgyre was protected by a protective shield of Warp-fire known as "Aetheric Fire", making any approach impossible. However thanks to advanced technology provided by the Adeptus Mechanicus, the warships of the Sons of Medusa were able to penetrate Wormgyre's shield of Aetheric Fire and launch a direct on the Hellgrammite's center. In the ensuing attack in which the Xenos were caught by complete surprise, the Hellgrammite homeworld was reduced to ruin and the survivors of the race itself scattered throughout the stars.