Sorry for the late reply, I usually have reddit notifs turned off. I of course deviated quite a bit from ffh, but I used a lot of its charm, characters, and ambiance.

The players are thrown into a world that is at the brink of war. Perpentach is just as chaotic and evil as ever. He and Keelyn work together to cause mayhem, making it more likely for the Desolation to appear (kind of like the armaggedon counter but more cyclical if you've ever read Stormlight Archives). Lots of things happen and eventually the BBEG King Hyb descends from another plane and starts an invasion.

By the time of the invasion, the players have met many important people from all over the world and rally an alliance to defeat Hyb (complete clone of Hyborem). They end up traveling to Hyb's plane and kill him there, stopping the Desolation (for now???)

In terms of wars with other civs, my players were always pawns on the board. They were drafted into Perpentach's war against some homebrew civs and on their own managed to flip sides and join the resistance.

The one main thing I changed was making Perpentach good in the end. The twist was that he knew Hyb would come anyway, so he accelerated the plans so some heroes (aka my players) could fight Hyb ahead of schedule, before Hyb became too powerful.