I was trying to mess with some Python files (in particular the event handlers), but I have no idea how I can define a scenario, in which, when a certain unit type is killed the game brings up the defeated menu. Ala the classical regicide.

The erie thing is -- I can't find a defeat menu/event anywhere. Only references to victory...

Any ideas?

You could always remove all units and cities of that player. Or give them to whoever killed the unit if non player nations can die that way.

Giving the cities makes more sense. I always hated how regicide destroyed the player rather than forcing them to be absorbed into your empire.

so is this going to add all new faction leader models if you complete it?

You could always remove all units and cities of that player. Or give them to whoever killed the unit if non player nations can die that way.
That's an interesting point -- I'm gonna try a few things and see what happens. Although I think it might be really cool if the entire nation turned into a barbarian state. :)

so is this going to add all new faction leader models if you complete it?
Think of unit formations with a single unit instead of 3 -- and that individual unit with a unique name and skin. That is more likely than a new model at this point -- considering no one knows how to import new models into the game as of yet.