I'm not so sure that Garrosh will be redeemed or convinced to chill out or any of that, but I won't say it's impossible either. First reason for that, he's one of the catalysts to the "putting the war back in Warcraft" thing that most people have wanted for so long, and the writers have decided was necessary.
Two, Thrall needs Someone to butt heads with or he becomes just a figurehead character. He exists, but you stop caring after awhile. There's obviously a ton of corruption in the Horde and plenty of people going against his vision, but you can't fight an idea or a concept the fun, literal sense. Thrall can't fight against the Orcs that want to return to the old ways in a way that players will find exciting unless there is a face to that concept. Garrosh is the embodiment of the Orcs that long for the days of the Old Horde. A struggle against Garrosh is a struggle for the direction of the New Horde. By overcoming Garrosh, Thrall would take another step toward reforming the Orcs, and we'd experience it in a way that's exciting, not just propaganda and other boring stuff that nobody would care about in an MMO.
On the other hand, Thrall's losses are a gain for the Orcs that want to be savage murderers. I think Thrall's loss in Orgrimmar kind of drives that point home. Thrall the modernist more or less lost his duel with Garrosh, and that's leading to a rise in the 'old ways' among the Horde. Thrall lost the battle, but he hasn't lost the war. That sort of thing. It's an ongoing struggle, with Thrall and Garrosh representing the two halves of the Orcish Horde. Right now, Thrall's losing ground. They haven't lost yet, but it's gotten to the point where a civil war is looking like a real possibility.
This one has been nagging me for a while: What the hell is the Scarlet Onslaught doing with Death Knights in their ranks?
This little stretch of story is way too cool for me to spoil just yet, so my advice is to just keep questing. All will be revealed in time. Let's just say the Crusade doesn't really learn from past mistakes.
So what exactly happened at the end of the Death Knight starting quests? There is something under Light's Hope, Darion sees daddy and turns on Arthas, Tirion tries to bubble but Arthas uses Death Grip, then Tirion somehow cleanses Ashbringer and punches Arthas in the face releasing only the DKs of Acherus.
The two most important things to remember are that Light's Hope Chapel is Holy ground due to whatever is beneath it (we don't know for sure yet), and that Ashbringer almost reflects the life cycle of a Naaru. It might actually be crafted from a Naaru but that's still speculation right now. The point is that it mirrors the same life cycle. It's a blade of Light, until it's been 'killed' at which point it drops to a blade of Shadow/Darkness. When it has had time to heal or is pumped full of holy power (iike you see in Old Hillsbrad), it is 'resurrected' and becomes a blade of Light again.
Light's Hope Chapel being Holy Ground weakened the Lich King himself. He was using the Death Knights of Acherus as fodder to draw out and weaken Tirion Fordring, because he couldn't take Tirion out himself on that spot. When they failed, the Lich King showed himself.
I think the scene between Darion and Daddy was a flashback the Lich King used to play games with Darion, but that's just my guess and this part felt really out of place and corny to me, so who knows? Personally I like to pretend this part doesn't happen, because man is it awkward.
The Ashbringer itself was rejecting Darion throughout the fight, so whatever is under Light's Hope Chapel started to turn the sword back to its Light side. It wouldn't obey the darkness anymore. Darion throwing it to Tirion Fordring, one of the current most devout/powerful Paladins in the world, that was the final turning point for the blade. The Ashbringer became Light again and let Tirion fight back.
The way the Death Knights of Acherus were freed is a lot like how the Forsaken were freed. A group of Scourge were given some amount of free will again during a point in which the Lich King had been weakened, and once they were aware of what was happening they weren't so easily swayed back into the ranks, especially with a strong-willed person to rely on such as Sylvanas or Darion Mograine. A lot of the Death Knights probably still hear the call of the Lich King, but know their loyalties now and can resist... if they want. They're mor or less just like the Forsaken. The big difference is the Lich King liked them better so most of them haven't had their faces rot off yet.
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