The worst period in human existence so far is probably the Toba Catastrophe - https://en.wikipedia.org/wiki/Toba_catastrophe_theory

However, our worst days are in our future and coming up really soon now. The edge of the climate catastrophe is already on us, and our response is just to pump more CO2 into the atmosphere. Once we're in the middle of it, every year will be as destructive as 536, except worse, because there won't ever be a respite.