And in the end, all it took was shoving all my +Nature items into the hands of one of my mages, turning 50 astral pearls into nature gems and casting Living Castle just before one of my throne sites was overrun.
Good game all!
Shout-out goes to one of my Kings of Fire, who submitted to astral magic enhancement and wearing that stupid-looking astral beanie in order to forge the +Holy artifacts so I didn't need to rely on Bishop Fish to capture the last throne I needed.
Edit: Overall, I'd sat that Kathan won due to a combination of being ignored until it was too late and sheer averageness. Our mages weren't really exceptional in any one path, and their mix of Air/Water/Earth/Nature, while providing for an amazing diversity of support spells, lacked any really good battlefield punch (with the possible exception of Rain of Stones, which I probably should've used more than I did.) Our troops were cheap and had decent HP and okay armor. One-on-one, they were pretty much universally unable to stand against anything that the rest of you fought with, but my capital was able to build 25 of them a turn, and most of my other fortresses could build between 10-20 of them a turn.
Although we weren't a blood nation, we did have a single hero that started with 2B, and one of our cap-circle provinces had a slave market. That was enough to start slowly trickling out Demon Knights, which would serve me well, especially against the flamethrower cavalry of my subterranean neighbor. Eventually we got enough 10% blood Camazotz that we had enough slaves to start calling in bigger things, but for the first 3/4s of the game it was pretty much just that one hero (and later a blood-enhanced mage) casting Demon Knights on repeat.
I'd designed my pretender to take advantage of one of my scaling national spells, planning on having masses of Shard Wights to help my armies. Unfortunately, even with Camazotz searching far and wide, we never really had enough death gem income to take full advantage (although I had some fun getting 40 or 50 longdead horsemen per cast of the Pale Riders spell, and much, much later, playing with tartarians.)
But yeah, mostly I attribute my win to the fact that none of you overworlders seemed to notice or care that I took over the entire underground, at which point I had a massive economy and a solid defensive position to work from.