You've been lucky. One of the AI civs must've explored a lair and got a "Spectres haunt the area" result. If one this spectres ate a few units, it could then upgrade to a wraith, and then... Well, yeah.
Nice to know. Though not that there is much you can do if it happens that early.
I could let autoplay run sometime and see how many civilizations that wraith manages to wipe before is killed. Though would be even more fun to see if barbarians win the game