Building my sci-fi setting deciding what's the in-universe answer to the Fermi-paradoxon, the reason why the universe still seems to be empty of K3 civilisations despite its age, has been quite important to me. I've settled on the notion that a species with a "functional setup" on a "suitable world" arises once per billion years per billion stars, but that a series of strong filters keeps everyone else from building galactic empires. Mankind is the only great civilisation in the next dozen superclusters. The enormous distances of space and time make seeing other civilisations in this early age of the universe impossible.
I have most of these filters pinned down, but the last filter should be that FTL is a suicide pact technology. This means that while FTL is theoretical possible in my universe, as soon as you use it to violate causality you are toast. I've encountered this idea elsewhere before, but the suicide part was either way too strong (the entire timeline where causality was violated stops existing) or way too weak (only the researchers and equipment working on the FTL project and all the data implying that there ever was such a project and such people stops existing).
I want a "causality correction" that wipes out a possibly already interstellar civilisation for good, but leaves ruins, artifacts and enigmatic data for my protagonists to find. I'd love it if the apocalypse would be spooky and enigmatic. However it should not wipe out a civilisation which has already spread to several thousand star systems, although it is acceptable if the event destroys hundreds of systems of such a civilisation. What exactly happens?
The setting is supposed to be sci-fi as hard as the narrative can take, but this is my little bit of Lovecraftian horror in the setting. Causality violation shall be a spectre that is enigmatic, unproveable and devastating.
The best answer will survive a meeting with Occam's razor, yet still give me the desired horror aspects.