Trying to think of some "epic" things that could be added to the late game to make it more interesting. They certainly wouldn't all happen in one game, and every game might have one or zero of them, but they'd really add some spice to the game and create more "OMG AWESOME" moments.
Desperate Bid for Victory
An AI faction that is losing a war casts a desperate spell that warps all of its champions, trained units, and even regular citizens into monsters. All units get huge (+20 to +50) bonuses to attack, defense, and hp as well as a +1 bonus to movement. Their cities cease to grow and cannot create any new buildings. Faction will not surrender, will fight to the last man.
Zombie Apocolypse
Well, Butchermen anyway, they're close to zombies and fit in the lore. Large, powerful Butchermen armies overrun the world, and any unit (including figures, not just full units) that die appear as a butcherman with their original stats. Possibly even champions. They're drawn to cities and group up to attack the nearest one. This keeps on going until a level 4 or 5 quest is undertaken to stop them.
Corrupted Land
An ancient curse is awoken that starts sapping the fertility from the land. It spreads from the source, corrupting additional tiles each turn. If it spreads to a city, that city loses 10% of its food per turn until its down to 10% food. This keeps on going until a level 4 or 5 quest is undertaken to stop it
Wildland Expansion
A wildland with one of the epic guardians decides to start expanding. Powerful invaders (almost as powerful as the main guardian) spawn in the wildland and invade the neighbors.
Dragonswarm
Dragons. Lots of them. Everywhere. Like the description when the player wins the game with the quest of mastery, but against the player and the AIs, and they have a chance to defend against it.
Corrupted Shards
All shrines are destroyed, and creatures of elemental horror arise from them and start devastating the land
Shard Domination
A new channeller arises and uses a weakened version of the spell of making in the attempt to bind all shards to him. All mana generation ceases, and players and AI start losing some % (2-3%?) of their mana per turn. Ends when the channeller is killed.
Thoughts on game-changing events? Other ideas?