The Cataclysm that was used to cleanse the world may not have been one thing; it may have been two things that were not experienced at the same time since the people and races were so far apart:
1. A nuclear explosion-type event (war of the gods? Blackmoor?) in this D&D fantasy world with accompanying nuclear winter (evil winter!);
2. An Ice Age (scientific or evil in origin);
3. Something else that causes fires and winters.
Okay, so that's three things. Anyway, to distract you from my inability to count, here's an interesting passage:
The First People of Tokpela, the First World, were safely sheltered underground as fire rained down upon the earth. Volcanoes and fire storms destroyed all that was above them until the earth, the waters, and the air itself was all elemental fire.
While this was going on, the people lived happily underground with the Ant People. Their homes were just like the people's homes on the earth-surface being destroyed. There were rooms to live in and rooms where they stored their food. There was light to see by too. The tiny bits of crystal in the sand of the anthill had absorbed the light of the sun, and using the inner vision of the center located behind the eyes they could see by its reflection very well.
Fire in the Jungle sourcebook by dbrandt regarding ant-based dungeons. Or you can use the compressed version of it on his blog.
Which reminds me, the sourcebook itself had a whole bunch of material on postholocaust jungles and the blog has a multiple posts on the matter. Time to read the book again.
In the meantime, let's reflect on the image of a giant monkey head (the Tomb of the Monkey God) and think about the possible cataclysm.
A war of wizards is mentioned in the FITJ sourcebook -- perhaps something greater? If I take this for Enigmundia, then maybe when the War in the Heavens took place, way back when arcane magics and divine spells were the same thing, it was not fought between gods but also between their followers -- the most powerful, skilled, and trusted sages and mages and warriors gifted with ancient lore.
I also like the idea of two distance empires fighting over the Jungle for some unknown reason, and the use of a Beam to control the peoples in the Jungle, and the transorbital lobotomy used to overcome the effects of the beam -- many many mysterious things in the past to uncover if the players wish to.