Tyranids - Assault: Once the Tyranid forces have established a foothold on the target planet's surface, they quickly begin the process of eliminating any resistance. Endless swarms of the smaller bioforms -- including Gaunts, Gargoyles, and Genestealers -- rampage through the skies, across the land, and under the oceans. Interspersed with these creatures, the larger bioforms -- including Carnifexes, Tyrannofexes, and even Hierophant Bio-Titans -- accompany their smaller brethren, sowing endless destruction across the planet.These varied breeds come together in large swarms, that sweep forward to create battlefronts that may be hundreds of kilometres in length. Behind the battlefront, Capillary Towers, many thousands of metres in height, burst forth from the planet's crust. These towers consolidate the biomass that the swarm has already absorbed, using it to breed additional warrior organisms to assist in the assault.While the towers may appear to be a Tyranid plant-analogue, they are no less aggressive than the more mobile bioforms. Capillary Towers extend tendrils throughout the target planet that can absorb indigenous species -- both plants and any small or slower moving organisms. As they do this, the towers also filter the planet's atmosphere, absorbing the biomass of airborne microbes and small flying creatures.Along the battlefront, Termagants use their ranged weapons to lay waste to opposition, while Hormagaunts boldly engage any opposition in close combat. These xenos, along with other variants of the Gaunt genus, may be found in swarms of hundreds coordinated by Tyranid Warriors.In those instances when sheer numbers prove inadequate to overcome the planet's defences, the Hive Mind may direct larger strains to assist in the battles. For those planets with extensive defences, the swarm may grow exceptionally large creatures. Specimens of the Dominatrix and the Hierophant strains have been observed which are comparable in size and strength to Imperial Titans.In cases when a defensive point seems insurmountable, the Hive Mind is quick to adapt its strategy. In many cases, Raveners and Trygons may be used to burrow under defenders, attacking from behind or beneath enemy lines. In other cases, Shrikes or Harpies may make strafing runs from above.While the swarm will readily sacrifice thousands of bioforms to take a single metre of ground, it will not waste their biomass needlessly. Alternative strategies are consistently employed, as the Tyranids are clear masters of evolution and adaptation. It is worth noting that the battlefront swarms that cross the target planet's surface do not typically devour the planet's defenders. Rather, they leave a swathe of refuse in their wake to be consumed by the next wave of warrior organisms.This carpet of the dead may, of course, also include those Tyranid organisms that the defenders have managed to overcome. Their deaths hold little meaning to the Hive Mind. They have served their purpose, and will serve the swarm once more after their corpses are devoured.In the skies over the prey planet, Gargoyles and Harpies swarm in numbers large enough to block out the light of the system's star. While they may assist in land-based battles, they also serve two additional purposes. Like the Capillary Towers, these flying organisms devour the biomass of the skies, returning it to the Tyranid swarm upon their deaths. They also serve to block out the sunlight from any photosynthetic life.Over time, this can have an effect on the planet's atmosphere and climate as the native plant life is unable to synthesise oxygen. The only effective recourse at this stage of the invasion is all-out war. By this stage, the invading Tyranid forces have firmly established a presence on the target planet. Eliminating the Hive Ship would not avert the damage already done.Further, depending upon the numbers and types of bioforms on the surface, the Hive Mind might still persevere. Even if the Hive Mind were overcome and the majority of the xenos were eliminated, the battles to cleanse the planet completely could continue for Terran centuries.The Tyranid genetic taint extends to the microbial level, and all of these xenos bioforms are strongly resistant to biological and chemical toxins. Even the tiniest Tyranid organism remaining on a world could have the potential to spawn more dangerous organisms given enough time and opportunity.