Snape's hand trembled under the unbreakable oath he had taken. How could he kill the only man that had ever trusted him? The old wizard's eyes drifted up, and from behind that crooked nose, he saw a familiar twinkle.
He felt a familiar tug at the edge of his conscious. He sought out the intentions of the tug, and finding good, he opened his mind. The Dark Lord had no good intentions.
"Severus", a voice exploded in his mind, blocking out all thought, " You must do it now. I'll explain everything, but not now. We have little time."
"Albus, I can't," he whispered.
"You must! This is the only way, Severus! Harry will never realize his full power if I am always there to protect him."
"I...I can't."
Tears formed at the back of his eyes. Not Dumbledore.
A spell slipped through Dumbledore's lips, soft as an evening flight, unheard by the surrounding Death Eaters.
"Impartio."
Again Dumbledore reached into Snape's mind. Now push Malfoy away and say the words "Avarda Kedavra." As Snape ushered the binding spell, Dumbledore again whispered a counter curse.
Instead of killing him, like the spell's brother "Avada Kedavra" this spell severed the connection between body and spirit so that when the curse took effect the body was killed, but the spirit was not. The spirit was transformed into a phoenix and stayed in the body according to it's will. Much like Voldemort's horcruxes, the spell saved the great wizard's soul.
Dumbledore would watch over Harry, and when the time was right, the once great wizard, Albus Dumbledore, would return.