The Tyranid Fleet moved into orbit over the planet releasing it's swarms of bioengineered ships towards the meager space defenses.

The Hive Ships did not wait for them to be destroyed.

Carrier spore began pouring to the planets surface carrying billions of Combat organisms within them. The spores will crash all over the planets surface in seemingly random locations. 

Few spores feel within the city itself but those that did carried more Genestealers and a new creature. The Lictor, pretty much a larger quicker deadlier Genestealer with cameleon type skin. 

Outside the city the swarm forms. Billions of organisms moving as one towards the city.

Something else far more insidious was taking place across the entire world. Micro Organisms were being released everywhere. These organisms would mutate the local flora into a rapidly growing Tyranid flora within a few hours. Huge Capillary towers will begin to sprout all over, they would change the atmosphere and raise the core temperature of the planet. Any creatures unlucky enough to have proper filtration would begin growing I'll.