Even without the Xeno-occupation, the Earth was due for one of the mass extinction events that characterize its long-term history. Massive volcanic eruptions and a severe ice age would dominate the globe for the next 80,000 years
The water that had been sequestered by the Xenos was released following the Retribution, and this sudden reversal in hydrologic fortune triggered a cooling trend that fed-back upon itself to become a freezing trend. The Yellowstone and Iceland super-volcanic eruptions were merely sideshows.
Early on, Antarctica became uninhabitable, and the remaining humans were evacuated, some to Venus, some to farther points in the Third Imperium.
At the height of the Eighth Ice Age, aka the Cryocene, glaciers extended down nearly to the tropics, obliterating any rubble that might have survived the Xenocene. In mid-winter, the Earth looked like cue-ball with a thin stripe of green and blue around the middle – the equator.
Human civilization on Venus persisted for 25,000 years, counting from the first scientific outpost at the end of the Third Fed to the final collapse – which we are getting to.
The terra-forming project began as floating installations in the high Venusian atmosphere creating graphine from the thick CO2 atmosphere for use in constructing the orbital that would become known as Ishtar. This proved so successful that larger, more advanced installations, called sky-castles, were made from the same graphine. At the peak, over two thousand such castles floated over Venus. In spite of their efforts, though, 500 years later the overall atmosphere was still 90% of it original, crushing thickness.
Towards the end of the Long Night, Prime Viceroy Glorious Howl pursued more aggressive means, directing several comets to be re-directed and smashed into Venus, hoping to accomplish – something. What she accomplished was the destruction of over a thousand sky castles, and a poisoning of Earth-Venus relationships that would last millennia. He successor halted the program (and his successor was the Overlord).
While the Terrans distracted themselves winning and losing far flung interstellar conflicts, the Venusians rebuilt with better technology, creating an artificial magnetic field – which had the side effect of shielding all of their communications. So when the Xenos invasion came, Ishtar was lost, but the sky castles floated undiscovered and unmolested.
Even 90 atmospheres of CO2 cannot last forever, and eventually, Venus was becoming too Earthlike to support the sky castles. Some Venusians left to join the Third Imperium (which by this time had liberated Earth), but some chose to stay, and see what could be done on the shores of the shallow seas their ancestors had once only dreamed of.
With star-age technology, the NeoVenusians chose their forms and inhabited the lush jungles and glades filled with giant insects and jewel-like fruits. They dapted so well to their faux-ecosystem that they forgot about the stars, content to cavort and frolic in their hidden paradise.
So they didn’t really notice as their artificial magnetic field degraded through lack of maintenance. Nor did they notice the solar winds blowing away their now far thinner atmosphere until it was far to late. By the time they were rescued, the last Venusiand were living in underground shelters, while the planet outgassed CO2 at a frightening rate, making the atmosphere hotter and heavier every long Venusian day.