BrainStorming Help!

[nocturne void]

I'm closing on the grande finale of my campaing and I was wondering if there were some people who would like to help me think things through.

Things I'm pondering:
Good mechanics for bigger combat / war.
I have a foggy idea of zoom in and zoom out method. So the players can see the war going on around them if they use maybe reaction or bonus action to perceive what's going on otherwise they are engaged in the immediate danger in front of them. I would use the success / failure of the players to see what happens also in the war around them.

Plot idea atm:
My big bad evil guy (Guras) has succumbed to the will of the god of death. He is wearing a mask called Face of Krobashi (God of death) and the mask eats his spirit little by little trying to control him, so the god of death can use him to wake up the titan / demigod sleeping in the volcano. And Guras wants vengeance on the queen of the tribes that's what sparks the war. I have thought that as they fight the BBeG on the verge of death the mask takes over him and causes the volcano to erupt and as they kill the mask creature they finnish the campaing. But the volcano eruption will cause a long winter to the summery island if we ever continnue this campaing.
Does this idea sound okay?

[cosmic carbon]

So if I understand correctly, the players fighting Guras is pointless.

If they fight him and win, the mask consumes his life force, the volcano erupts, and then the winter happens.
If they fight him and lose, the mask kills Guras and the volcano erupts anyway.

Why should they fight him if nothing changes?

[nocturne void]

Damn. Kinda true. If they lose, Guras will take over the leadership of the island. 🤔 But not a too big reward there for winning.

And if they lose the mask won't consume hum immediately. Maybe later, I was thinking it would be a cool 2nd level for Guras to Be consumed and transformed to a monstrosity.