Sorry for bumping this thread but I've given this another playthrough and something happened. Something that left me just sitting there in astonishment. I've made screenshots. Look:
This ain't battle screen.
Ok...
OK! Wait... what the hell?
Balor is my champion!
And now he's the province leader!
The final battle at Tory Island. Balor is not present.
How I did this? As you know Balor invades one of the northern coastal provinces once all other tribes are conquered or allied with you. I've had the last enemy tribe occupy such province - to be more specific province no. 2 (the one in top-left corner). I've turned this tribe into my ally, you know - when they assume blue color on the map and you can't attack them anymore and, given some time, they decide to submit to you completely. This tribe's ruler (Sinnan Og) came to announce hers tribe submission. However in the meantime Balor devastated the province. As you also know the game forces you to attack the province immediatelly. This is all a kind of a normal scenario. However this is where things went wrong. Normally, when Balor invades a province, all champions inside are destroyed, however Sinnan Og wasn't in the province at the time - she was on the way back. Immediatelly after my champions put their feet in the province Sinnan Og appeared and said "I can't win, therefore I have to run away." The battle ended and I had Fomors under my command!
It's too bad you can't use 'em for anything. You can't even bring them to the final battle. On the other hand Balor is not present in the final battle and to win you just need to defeat Domnu.
I guess such course of events wasn't predicted by the developers.