

spoilers for a number of works follow
Hyperion - mankind lives in uneasy competition with semi-hostile AIs. Wars between human factions have already killed billions. The Earth is destroyed by a science experiment dropping a black hole into the core. The farcaster (teleportation) network causes ecological disasters on multiple planets.
Dune - after catastrophic wars between humans and AIs, computers are forbidden. The only way to travel interstellar distances are via the monopoly of the Guild navigators. Society is explicitely feudal. Our hero protagonist disrupts this, establishing a theocracy in a war that kills billions. His successor holds humanity in societal stasis for millenia, to induce the Scattering that will preserve it from similar societies in the future. Of course, billions die during this period.
Foundation - humanity collapses into a new Dark Age. Presumably, trillions die.
Ringworld - I read it long ago but it was so 70s I’ve basically blotted it out. The wiki summary indicates it’s not so bad unless you’re stranded on the Ringworld itself.
pls let me know who thinks Ringworld is ”hard SF” so I can punch them