Sorry, BloodLily16, but I will not be telling you who it is.
At this point only the Doctor knows who it is, and since he is a little 'busy' right now, the only way that you will find out is if you either guess correctly, the Ponds find out or I just tell you.

And we all know I'm not going to tell you...

So far, nobody has guessed correctly, but those who have guessed have made very good guesses. So far, some of the guesses have been-

1. The Mara
2. A victim from Gallifrey
3. The little girl from the 'Family of Blood'
4. The Slitheens
5. The Nightmare Child

The Ponds

Amy watched with blurry eyes as the Doctor, trapped on the other side of the glass, was confronted by black smoke. The smoke, however, stayed back, like some invisible force was holding it at bay.

"Doctor!" She yelled, hoping he could hear her through the glass, completely forgetting that even if he could hear, he couldn't understand her. The Doctor turned back to the glass and started frantically hitting the glass with all his strength.

Behind Amy, the little girl sniggered and the black smoke rushed at the Doctor as if nothing was holding it back any more. It surrounded the Doctor and the Ponds could barely see him as he pulled out a handkerchief, cover his mouth and run into the glass as he turned around and tried to get away from the gas.

"Do something!" Amy screamed at River and Rory. River quickly reacted by pulling out her gun. Sher aimed at the glass and as she pulled the trigger the gun malfunctioned in her hand, causing it to heat up and River dropped it, burning her hand and exclaiming.

"You answered wrong." The little girl said as she walked up to the glass. "You played my little game and you lost. Now he is going to die."

"Oh, god." Amy whispered under her breath.

"You want to save him, don't you?" The girl asked, a stern look on her face. "Why would you ever want to save such a creature as that?"

"Because he's the Doctor!" Amy yelled, trying to get closer to the girl. Rory held her back and she fought for a bit before giving up.

"I don't think that qualifies as a reason." The little girl said as she started a restless pace. "I want a specific reason why I shouldn't kill him now. And I would hurry if I were you, time is running out..."

"Because he's saved hundreds of worlds and millions of people!" River augered back.

"And he's killed thousands more." The little girl said darkly. "People like that don't deserve to live."

"What makes you better than him then?" River questioned. The little girl got closer and the group backed up until both Amy and Rory were pressed against the glass wall. The little girl looked deeply into River's eyes, completely silent. She turned and walked away from River after what seemed ages.

"If you want to save him, you'd better hurry." The little girl said as she walked away. Rory turned his head to the Doctor, who was on one knee, struggling not to breathe in the gas. He turned back to the little girl to find that she had vanished in a puff of smoke.

"She's gone." Rory said as the three ran to the Doctor about ten feet away. He was now on both knees and soon fell on his back, chest heaving as he struggled to make his lungs work. Rory frantically looked around for something to break the glass while River knelt by the Doctor's head, tears welling up in her eyes.

Rory began to pound on the glass, overwhelmed by the feeling of dread in his stomach. "Come on!" He shouted, trying to break the glass. Below him, the Doctor laid, unable to move expect to pitifully try to pull oxygen into his lungs.

Rory paused in trying to break the glass and looked down. The Doctor had stopped moving but his eyes were flickering about, but mostly staying on River's face. He took in one final breath before his eyes rolled back into his head and he was completely still.

Amy sprang from where she was crouching right into Rory's arms, sobbing. Rory rubbed Amy's back comfortably, shhing in ear as he felt tears from his own eyes fall.

River sat back from where she was crouching with a cry. Gingerly, she reached an outstreghted arm and stopped centimeters from the suface of the glass.

Fighting back tears, River put her palm on the glass.

As soon as she touched it the glass cracked, a spider web pattern forming from the center of her palm and spiraling outwards. All three of them gasped and the glass shattered into a thousand pieces.

Rory pulled Amy close and covered her as best as he could with his arms as the glass fell.

After all had settled and was calm again Rory looked up at the damanage.

Sharp shards of glass covered the floor and glittered when the light hit them, but other than that, the room was completely empy, save for the Ponds, River and the unmoving Doctor.