536

ok super cool idea:

volcanic winter of 536 caused by major volcanic monster awakening, causing eruptions worldwide that causes all that bad stuff. world leaders at time come together in proto-GOC/Foundation to deal with it. Justinian I, some mesoamerican guy, nordic dude, chinese, minoans, gupta. they all make pacts with several beings to help deal with this event, but in exchange pay with worse events that don't last as long (plague, fall of several empires, etc)