Thanks Guys! It make a lot sense now. I've been doing a lot of side quests and neglect the main quests. I'll go back and push forward the story and get this brand ability, seems like fun.

I just don't understand how this thing work and what's their impact to the game.

I've killed a few captains, and others soon take their places, I don't see how I "weaken" Sauron's army. And often times, captains I already killed came back to life and I had to fight him again. Killing a captain doesn't seem to have any impact of the game world, there's still endless respawn of enemies.

I don't understand the meaning of power struggle, and the game doesn't explain it very well. Could someone help?