The Knight Mechs and Fleet Iron Guard reached the Equator, entering the Tyranids' ecosystem.
The Fortress World had once been covered in Concrete, with bunkers and trenches everywhere.
After the Tyranids arrived, they continuously released Spores into the air. Once the Spores landed, they began changing the local environment.
Purple fungal threads first grew across the Concrete. They released Digestive Enzyme that corroded the Concrete and Rebar, turning them into clay.
The threads rooted themselves in the clay and pushed downward until they reached the original soil, where they began absorbing trace elements.
Fed by those nutrients, the threads grew purple blisters several meters long.
When the blisters matured, they burst, releasing more Spores into the air and converting more distant land into a Tyranid ecosystem. That completed the first stage.
In the second stage, once purple fungal threads covered the land and the Tyranid ecosystem had begun to take shape, the Tyranids started building more advanced structures.
There were purple Digestive Pools several kilometers across, for example. Like resource deposits in an RTS game, they were used to reclaim biomass.
Then came the Hatching Chambers, the equivalent of barracks.
Next came Bio Anti-Aircraft Cannons several stories tall. They fired at fleets in space to prevent enemy warships from bombarding the Tyranid ecosystem.
The third stage was self-destruction. Once the Tyranids had taken the planet, they no longer needed the ecosystem, so they began the final process.
Every structure in the ecosystem was broken down into Nutrients and fed into Digestive Pools dozens of kilometers wide, where it became biomass.
The Tyranids' Bio-Ships descended as low as they could and lowered fleshy tubes hundreds of kilometers long into the Digestive Pools, drawing all the biomass aboard.
Once the pools had been drained, the planet would be nothing but sand and