What was there 65 million years ago?
If you asked an ordinary human from Earth in the 2K era, they might say there were dinosaurs 65 million years ago, and perhaps a man in a red bodysuit crouched inside a cannon barrel, blasting his way through the streets.
But Hogg could tell you clearly that the dinosaurs didn't matter. The most important things were the Old Ones and the C'tan fighting nearby. Perhaps the dinosaurs had gone extinct because the War in Heaven had spilled over and wiped out their planet.
In this world, the War in Heaven was an unavoidable subject. Just like its name suggested, it had turned the entire Galaxy, once a paradise, into a cesspit. And with both sides boasting power that reached beyond the heavens, even if you threw the Emperor into the mix, he'd have to kneel and call them "Daddy."
It wasn't just the Old Ones and the C'tan fighting each other during the War in Heaven. They were merely the leaders of their respective sides, with countless powerful species serving under them. Any one of those species would be considered a dominant power today.
Take the Greenskins, for example. The Orks of today were nothing compared to their ancestors. During the War in Heaven, Orks stood over ten meters tall and could take out a kid with a single punch. Even Guilliman would have gone out of his way to avoid them. And such powerful creatures were merely disposable cannon fodder in the Old Ones' camp.
The C'tan's side was even worse. The great masters of the physical universe had joined the battle personally, and before they were weakened, their Necrons had been completely broken.
If you still couldn't understand, imagine converting every Astra Militarum trooper in the Imperium into a Space