Koristrasza is back in the instance (now in dragon form) and explains the changes to the instance. She also assists you on the way to Amnennar by shooting down fireballs that turn into helpful healing circles for you:
I too thought Amnennar the Coldbringer had been defeated; a grave mistake indeed. How clouded my judgment must have been to forget the true power of a lich lies in the phylactery.
A powerful necromancer such as Amnennar can store his soul in a physical vessel, allowing himself to be reborn if the proper rituals can be completed.
I should have searched across Kalimdor for Amnennar's Phylactery, but I was too prideful in our previous victory. Now, all of that work may come undone.
I know Death Speaker Blackthorn's newest creation, Mushlump, spews forth plague from every pore and he has reenlisted Modresh Fire Eye's help in rallying the undead.
Even the Death's Head has been helping Blackthorn. Their Aranomancers have created a monstrosity of fang and plague, a spider of enormous size named Aarux.
If you could defeat these enemies and clear the plague blocking my path we could climb the Spiral of Thorns to defeat Death Speaker Blackthorn and recapture Amnennar's phylactery before it is too late.