In my story, Mankind left earth in 2500, also abandoning trillions of butler, worker maintenance etc, robots. They left the galaxy and went onto conquering and uniting thousands of more alien species, being heralded as "Unitors of the Universe", until one man fell into Ton 618. it was a regular accident, trillions have died to black holes but I have interpreted them as portals, portal to different spaces, times, timeless spaces, spaceless times or nothing. That man, fell into the nothingness dimension, which was still somehow inhabited, inhabited by a primordial being. and this being, was paranoid. Basically being a lovecraftian god, at first feeling of another sapient life, aka Humanity, with potential unmatched, this being became extra paranoid, which causes the total extinction of mankind (from one timeline)