Gryphonne IV - Hive Fleet Leviathan: Less than five standard years after the defeat of Hive Fleet Kraken in the Second Tyrannic War, Inquisitor Lord Kryptman noted the tell-tale signs of yet another Tyranid invasion in 997.M41. He implemented the Kryptman Census, burning out dozens of astropaths in the attempt to contact scores of worlds on every fringe of the Imperium. Slowly, the responses -- and lack of them -- formed a pattern, and the venerable Inquisitor was able to chart the course of the Tyranids' latest Hive Fleet to arrive in the Milky Way Galaxy.Codenamed "Leviathan," this time the Tyranids were approaching from below the galactic plane, attacking from two points spaced wide apart in the shape of a closing pair of jaws, cutting off large portions of the galaxy from Warp travel or astropathic communication. At a great cost in human life, a combined force of Planetary Defence Force troops, two regiments of the Astra Militarum, the Ultramarines and Mortifactors Chapters of Astartes, and a Deathwatch Kill-team under the command of Inquisitor Kryptman crushed the left "half" of the Leviathan on the world of Tarsis Ultra, destroying the Tyranids nearest to Terra, and reestablishing astropathic contact with the worlds between the Leviathan's jaws. The Imperium had won its first victory.Yet the Leviathan continued to carve its bloody path through the Imperium. Perhaps the Leviathan's main gain was the vital Forge World of Gryphonne IV, home of the Legio Gryphonicus Titan Legion. Kryptman knew he had to slow down the Hive Fleet's advance to buy time for the Imperial Navy's Battlefleets Solar and Tempestus to muster.With grim finality, he ordered a cordon to be established. Every world within was to be evacuated and undergo immediate Exterminatus wherever possible, thus causing the Tyranids to expend their forces for little gain, the Exterminatus actions denying them valuable biomass to replenish themselves. With one stark, callous decision, the Inquisitor had inflicted the Imperium's worst act of genocide upon its own population since the Horus Heresy. Kryptman was denounced as a Radical and a Traitor; when migrating Orks claimed a score of the former human worlds, he was stripped of his title and cast from the Inquisition by his brethren.Over the millennia, the defences of Gryphonne IV were built up until they rivalled those of Mars itself. So formidable were its fortifications that the Tech-priests refused to abandon their industries even when it become clear that the Forge World lay in the direct path of Hive Fleet Leviathan. Instead, they shored up their walls, certain that the Tyranid waves would break against them. The Adeptus Mechanicus refused to abandon this important Forge World to Exterminatus.The Magi of Tesla Prime abandoned their world in 498.997.M41, choosing to use its military forces to bolster Gryphonne IV, one of the principal Forge Worlds in the galaxy, in preparation for a defiant stand against Hive Fleet Leviathan. Isolated and alone, and left for dead in an abandoned star system, the Legio Gryphonicus prepared for war.Before the arrival of the Hive Fleet, Gryphonne IV boasted some of the finest defences of any Imperial world within the southern reaches of the galaxy. Gryphonne IV's War Griffons Titan Legion was feared for its world-breaking firepower and its Skitarii Legions, who were envied across the Adeptus Mechanicus for their discipline and precision.The armies of Gryphonne IV fought with consummate skill, yet the xenos invaders were unrelenting. One after another, the Adeptus Mechanicus guns were silenced, the defence lines shattered. The Skitarii were annihilated and devoured by waves of inexhaustible Hormagaunts and the Legio Gryphonicus was torn asunder by Tyrannofexes and Tyranid Bio-Titans. Within solar days, the world was scoured clean of all life. Gryphonne IV was ever after a Dead World -- a grim testament to the folly of human hubris and pride.The tale of Gryphonne IV does not end with a barren orb, however. When the outcome of the battle was beyond doubt, a handful of Tech-priests fought clear with a few battered cohorts, extracting vital blueprints with which to remake their lost Skitarii Legions.They vowed to build a new Forge World and continue the Omnissiah's work. Travelling across the torn Imperium, they seek a new homeworld. Even a few Titans escaped, but the Legio Gryphonicus's mechanised gods of battle were last seen fighting upon the Fortress World of Cadia against the Forces of Chaos during the 13th Black Crusade, their final fates unknown.Though the loss of Gryphonne IV is a calamity of unprecedented scale for the Imperium, the Tyranids were uncaring of their victory and Hive Fleet Leviathan simply moved on in search of fresh feeding grounds before its final destruction during the Devastation of Baal in the early 42nd Millennium after the birth of the Great Rift.