I don't own Twilight.
He felt him here. If Demetri had a heartbeat it would have been pounding in his chest. If he had sweat glands he will be soaked. Since Carlisle began assembling his army Demetri knew Amun will be there. They were only a few meters away. He remembered his creators look of despair when he decided the Egyptian coven was no longer his home. If Demetri and Amun were in this desolate field alone he would have explained everything to him. Why he had to leave? Why he belonged with the Volturi? Why Aro was everything to him?
Nothing he says will changes anything.
The ending in kinda Chelsea's effect on him. Aro made sure Chelsea tightened the bonds to him before the showdown.