A Xenos Inquisitor on Triturus II investigating a Genestealer cult first saw the signs of the coming invasion. He sent word to the world of Triturus IV once reports of converging Tyranid splinter fleets were confirmed. Once the entity known as The Swarmlord was sighted, an all out request for aid was sent for reinforcements.
The Tyranids hit planet fall well before their projected timeline, making the Planetary Defense Force to scramble their Army. With out fleet support, Full waves of Tyranids rained through the atmosphere to the planetary north. From their they made their way across the planet devouring outlying settlements that provided little resistance.
As they came closer to the population centers of the planet, the PDF's air forces attempted to keep them at bay with bombing runs. This forced the Tyranids to the tunnels and catacombs that lead to the great canyons. In these Jungle valleys, the air superiority would be limited, and cover was plentiful. It was at the convergence of these Canyons that the PDF would make their last stand.
Each of the two major Armoured divisions held a choke point of the canyons, with the swarming tide of Tyranids infesting their way towards them.
Multiple Astartes forces had received the distress call of Triturus IV, but the swarm had advanced to quickly and reinforcements wouldn't arrive in time.
Each point fielded an impressive array of Heavily armored tanks, including near 10 Russes, and 3 Baneblades each, as well as multiple artillery pieces. The Swarmlord countered such resistance with measures of his own. Two of the Gargantuan creatures classified as part of the Hierodule strain in addition two 4 mighty Tyrannofexes escorted the Swarmlord himself into one of the sides.
On the other side an ambush was set as Trygons spearheaded the assault bursting from the ground while zoanthropes, genestealers and Carnifexes burst out of secret passages of the canyon walls.
As the Lumbering Creatures kept up their assault, Russ and Baneblade alike fell before their crushing weight. The swarm that followed moved as a living carpet, clogging any hope for mobility or reinforcements.
By the time the Astartes fleets arrive to the Planet, it is too late. The entire planet had been infested. Oddly there seemed to be far fewer Tyranids present then to be expected of a planetary invasion of this magnitude. It almost had seemed as if the planet had been abandoned by part of the swarm during the assimilation process. But this was no matter to the arriving Space Marines, as Cyclonic Torpedoes were fired, fulfilling an Exterminatus order on yet another lost planet.