Now:
"See what I have become?" the face says to him. "Mere shadow and vapour ...I have form only when I can share another's body... but there have always been those willing to let me into their hearts and minds... Unicorn blood has strengthened me, these past weeks. You saw faithful Quirrell drinking it for me in the Forest... and once I have the Elixir of Life, I will be able to create a body of my own. ...Now, why don't you give me that Stone?"
The voice is soft and sibilant - and strained.
Harry watches him speak with colder eyes than he ever had when he was eleven. Or fourteen, or fifteen or even seventeen, for that matter. Voldemort is not a man of restrained passions, and Harry can see from Quirrell's shaking - for real, now, shoulders hunched against some great weight and muscles trembling with it - that it's getting harder and harder to withstand. That's no surprise. The horcrux he's supporting is only growing stronger.
"It's not here," Harry admits, leaning back against the mirror and sliding his hands into his pockets. "It hasn't been here all year. I have it, though."
The red eyes blink. Once, slowly. "Impossible," hisses Voldemort, and Harry can see how it's taxing him now, Quirrell's body heaving with each pause Voldemort takes for breath. Are they sharing lungs? Harry cringes internally. Gros. "Taking it out from under Dumbledore's nose... is a feat a grown wizard would struggle with."
"That would have been really hard," Harry agrees, "but I didn't take it from Dumbledore, actually." Truthfully, Dumbledore hadn't even handled it overmuch, which is a stroke of luck for Harry.
Now's the hard part. For all that this is what Harry's been aiming for all bloody year, he still doesn't feel prepared for it. He still feels like he has no idea what he's doing. The thing is - it's Voldemort. Even though Harry knows exactly how to defeat him and has clear evidence that he can defeat him, Voldemort is the stuff of nightmares.
He bites his lip, but then he straightens up. The decision's already made. He made it way back.
"I can get you the Elixir of Life," Harry says. "But I need your help with something else."
"You," says Voldemort, cautious now, wary now that this confrontation hasn't gone at all like he expects, a little incredulous, "need Lord Voldemort to help you?"
Harry looks right past him for a second, jaw tense. "Yeah. You're the only one who can, apparently."
Slowly, and with laboured gasps from Quirrell, Voldemort begins to laugh.