The Aeldari were one thing, but what had become of the other xenos that threatened the Imperium of Man?
As another psychic race, the Orks could no longer sustain their Waaagh! field after the Five Elements Overcoming took place. Fighters in midair, trucks racing across the ground, even the reactors they used to power entire settlements—all were damaged or fell apart once the Waaagh! field failed. The harder something had been imagined into existence, the faster it was destroyed.
Many Ork camps did not even need the Imperium of Man to send troops against them. The Orks were wiped out in reactor explosions. The Orks' greatest mekboyz had begun new research into the failure of the Waaagh! field, but before they could make any progress, the Imperium of Man would completely eradicate the green plague that had plagued the Milky Way.
Another xenos race was the Necrons.
To be honest, if not for clearing the battlefields around the portals, many in the Imperium of Man would never have known that such an ancient and terrifying civilization lay beneath planetary crusts.
Based on Neoth's explanation and records from ancient Aeldari texts, the Primarchs roughly came to understand the origins of the Necrons. They also learned that Ferrus's Iron Hands were actually the living metal commonly found among the Necrons.
The Necrons were a threat, if they were still alive.
After the Great Transformation, the Necrons died completely. They became true machines, trapped within their own logic networks, like tomb guardians protecting the remnants of their vanished civilization.
Now, the Adeptus Mechanicus was frantically sending exploration teams to excavate Necron tombs. Whenever a Necron awakened during surveying work, the Adeptus Mechanicus immediately retreated into planetary orbit and waited for several days. Having lost their hatred, the Necrons would return to their tomb