On one single planet:
Loyalists intentionally annihilate the entire surface with nukes after a rebellion
the survivors live in tunnels and ruins like rats
World War One-style trench warfare continues for five hundred years across the ash-filled wastes
after the war is over the planet’s society is basically a death cult which exists to be tossed into the meatgrinder, and they probably use child soldiers and weird test-tube-babies due to having so few people left.
And that’s just one single planet that happened to be Fallout. Anywhere the Tyranids attack basically experiences it’s own apocalypse. 40k is so gargantuan that if you can think of a horrifying situation it’s probably happened multiple times already.