you don't have to do a master quest to finish the scenario. there's one quest involving the gate to the gilden realm, but that quest line is optional - if you finish it, gilden joins in the war, but they are hardly required to win. in my campaign, the allied civs didn't contribute anything meaningful to the invasion - doesn't matter if there's one more pathetic AI stack moving across the map and doing nothing useful.
this will be a bit of a spoiler, so if someone wants to find out on their won, don't read on
SPOILER
when you're done with the western Yithril enclave (Yithril Invasion), it's time to build up your Invasion force to bring the war to the Empires. Once you cross the landbridge to the eastern part of the twin continent, Queen Procipinee tells you about the Forge of the Overlord which is located on the other landmass. You fight your way through Karavox' and Ceresa's lands and when you get to a certain location on the eastern continent, Queeny pops up again and tells you that the Forge is at the eastern side of the valley you just found. fight your way though it, find the forge, do the Quest in the Forge and game over. You can also kill off Magnar and Yithril first, but that's not really required. I thought it might be an optional/alternate end, but nothing happend when i defeated the last empire (that was the first version of the campaign in the 0.90 beta, though. don't know if this was changed in the meantime, didn't replay the campaign yet)
