What size object could wreak havoc on a world but not "destroy" it?

So what size comet/meteor/rogue planet could wreak havoc on a populated world without killing off intelligent life entirely or actually ruining the planet for life? Assume the world being affected is roughly the size of Earth, and the object is passing at a distance somewhere between the Earth and Moon. By "wreak havoc", I mean that the object would cause temporary climate change, floods, fires, devastation from impacts by smaller trailing objects, etc. Assume a roughly medieval/renaissance era human population lives on this planet - how big of an object would it take to cause enough problems to wipe out population centers and maybe 90% of the people and essentially revert them to a stone age society?

posted over 8 years ago

Skelter‭

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