The Laanah Rifts is a region of the Segmentum Pacificus located in the northwestern fringes of the galaxy which was long held in the hands of aliens and Heretics, until the Imperium launched the Belrath Crusade in 182.M38. Within a century-and-a-half, a great swathe of the Laanah Rifts was brought under the dominion of the Imperium of Man, which contributed to the eventual founding of the Laanah Sector.
The Laanah Rifts was a dangerous and largely uncharted area of space, much of which had remained untouched by human contact since before the Age of Strife. In the year 182.M38, under the auspices of Praetor Slayban Bellrath, a powerful noble from the Terran court raised to the rank of Lord Militant, a mighty crusade was instigated to liberate and dominate this long-benighted area of space. The Crusade met stiff resistance to its progress from the start. This region was home to the foul xenos known as the "Hellgrammite" -- anthropoid creatures that possessed occult-infused weaponry capable of flooding the void around their worlds with Ætheric fire -- and although their physical forms were decayed and frail, they rode to battle on the backs of howling bio-mechanical monstrosities whose barbed pincers could slash through tank armour with ease.
Within fifty standard years the Crusade began to falter, sustaining severe setbacks, time and again, at the hands of the twisted alien race. It was at this time that the Sons of Medusa Chapter would intervene and would win one of their greatest victories of the Crusade. With Bellrath's crusade salient routed and the horrific xenos harrowing the retreating Imperial forces, the full might of the Sons of Medusa circumvented the aliens' attack fleets and conducted a perilous counterattack, cutting behind the aliens' battlelines and striking like a spear thrust at the heart of their domains at the star system codenamed "Wormgyre."
With the death of Wormgyre, the Sons of Medusa tore the heart from the enemy, both militarily and spiritually, and the Hellgrammites' resistance to the crusade collapsed, their will to make war broken at a stroke. Within a standard century, a great swathe of the Laanah Rifts was brought under Imperial dominion, and the Sons of Medusa entered the mythology of the new sector's inhabitants, whispered of in the old tales as "the Emerald Death" that fell from the stars to punish the God-Emperor's foes. The Sons of Medusa Chapter, for its part, has retained some ties to this, the site of one of their most lauded victories, maintaining infrequent contact with some of the Adeptus Mechanicus factions that founded bases on its frontiers, and holding seldom-used recruiting rights on several of its worlds in tribute to their role in the Laanah Sector's founding.
- Imperial Armour Volume Ten - The Badab War, Part Two, pp. 97-98