Is that really one of the hardest thing to believe? I mean, hardest that conquering a Niefel fort by Caelum and suddenly all the population going from gigants to Wingedmen?
A powerful being can control a quite big land after he destroys the armies there. There will be some people unhappy (unrest) until you prove the that living under your government is better than previous condition (tax=0).
Of course there aren't historical examples, but just thing of greater acomplishments made by small numbers of people. 300?
Or conquering South America with a couple of spanish people
.