Sybill
In the original plot, the war started because of the rogues. The fight between the Lycans and the Rogues was devastating, it caused a lot of destruction and created a huge crack in the barrier that protected Zealandia from the outside world.
When the barrier was broken, the greedy eyes of the outside world started focusing on the magic in Zealandia. What they don't know was the fact that Zealandia had been sealed for a reason. Zealandians were the protectors of the world, the people who are fighting the demons that wanted to cross-realm and wreak havoc on Earth.
With the barrier broken, the demons immediately started roaming outside of Zealandia. By the time the world knew about the existence of demons, they were already everywhere, preying on the weak. Two years after the barrier was broken, a large group of demons crossed the realms and enslaved humanity.