What happened after the Creation War?

This question unfortunately only crossed my mind after Pat Rothfuss's AMA some time ago. Tell me whether I missed something.

So, in the first book, Skarpi tells the story of Lanre and the Blac of Drossen Tor. He says, "More people died at Drossen Tor than there are living in the world today."

Now, he might have said that for effect, but even if it was partially true, that means there were a lot of people in the world back then. If I remember correctly, when it comes to warfare, a mortality rate of more than 10% in any given battle is considered a bloodbath. Obviously the Battle of Drossen Tor was bound to be bloodier than most other battles, but the implication is that the world's population back then (5000 years ago) was immensely larger than in Kvothe's time.

For the population to fall so drastically, there had to have been a massive cataclysmic event that wiped most of humanity off the face of the earth.

I hope I didn't miss something big. What happened?