So...this is where it gets stupid, but it seemed like a good idea at the time. Sorry.
"Andy! Andy!" The Doctor hadn't stopped calling for her since the stone had fallen over the entrance. Matros sat on a stone near the cave, watching. At first the Doctor had been angry, demanding to know why Matros wasn't helping.
"It looked like something she needed to do for herself," was the reply. This had stumped the Doctor, and had even gotten him to sit down beside Matros for a time, until he heard screaming in the cave. Then he'd gotten his sonic screwdriver out, and had tried every way he knew to try and move the stone, even pushing it. Matros watched all of this, not amused, but at the same time not believing that help was needed. Finally the Doctor gave up pushing when the screams and rumbles from within the cave abruptly cut off.
"Andy! Andy!" There was silence, except for the Doctor's hurried breathing. Then the stone, which had resisted all of the Doctor's efforts, was lifted into the air. The Doctor immediately scanned the entrance of the cave.
"Andy?" he called, checking the readings. What he saw there had him staring, open mouthed at his device, and then up at the cave. A large in-take of breath came from Matros at the same moment. They'd both seen them. The large glowing green lights. They came steadily towards the two men. The Doctor stepped to the side, while Matros stumbled back. The sound of footsteps came from within the cave, and out came Andy. Or at least, Andy's body. Because the glowing green light was shining out of her eyes. Not just her irises, but what had been the whites, were glowing fluorescent green.
She walked out with purpose, as if she had never had a cause to doubt herself in her life. The way she held herself, she appeared to have grown about three feet since entering the cave. She was imposing. She was magnificent. But she wasn't Andy. She looked to Matros and regarded him,
"Thank you for staying," her voice wasn't Andy's. It was rich and cultured and rang with the sound of eternity. The Doctor got goosebumps just looking at her. Matros looked terrified. He clearly didn't know what to do. In the end he just nodded his head. She returned the gesture. Her gaze swept around to the Doctor. He shifted, uncomfortably. She seemed to be reading his thoughts.
"It was a foolish thing to offer, Time Lord. But you offered it to save her, and I appreciate that." He also nodded, but couldn't resist asking,
"Who are you?" She smiled. It was nothing like the Devouring Dark's twisted smile. This was a smile of kindness, and a little amusement.
"I am all around you. I am the tallest mountain and the tiniest grain of sand. I am Gaia." She said it simply, as if wondering why he hadn't guessed already. But in a day of extraordinary surprises, this was perhaps number one.
"The goddess of the earth," the Doctor said incredulously. Matros' eyes grew wide, and the Doctor could almost see his mind decide that it had seen too much today. He fell to the floor in a dead faint. Neither Gaia in Andy's body, nor the Doctor made any move to catch him. She looked at him, and he looked at her.
"Why are you here?" he finally broke the silence.
"To protect Andy," her voice was again kind.
"Where is Andy?" he worried.
"Sleeping. Here," she pointed to a space at the back of her head, "She has had a rather long and painful day, she could use the rest, don't you think?"
"Of course. I was just…is it gone?" he was worried that after everything she may have succumbed.
"Yes, the Dark is gone. It will not come back for some time." she glared into the darkness of the cave, and brought the stone, which was still hovering in the air, back down to rest in front of the entrance, effectively sealing the cave off.
"You can name it?" the Doctor said, uneasily.
"It has just been defeated, and by a young girl, no less. It will not be strong by any means."
"She really defeated it," he was amazed, for someone so young to have accomplished such a thing, it was unheard of. Gaia's eyes sharpened,
"Just barely, I should add. It tried to take her with it. It would have succeeded, had I not stopped it. Do not celebrate yet, Time Lord," her tone was crisp with anger.
"What do you mean 'it tried to take her with it'?" it sounded ominous enough that the Doctor thought he already knew.
"It tried to take her soul. I would not let it. Her soul is bound to me, and I will not lose her now,"
"But, why? Why do you care?" The Doctor was honoured to meet a full blood goddess, even if she was wearing a human body, but he didn't understand what she wanted with Andy.
"She's my daughter," Gaia replied.
Another second and the Doctor thought he might join Matros on the floor.
"She's what!"
"Well, technically she's my granddaughter, with several thousand 'great's placed between, but I always think of my Magis as my immediate children."
"You…you have descendants! But that means you have to have had…" He blushed when he realised what he was about to say. The bright green eyes looked at him with amusement.
"In my corporeal body, I am human shaped, and so have human desires. And just because I am a goddess doesn't mean I have never known love," here her eyes took on a sadness that was millennia old, "I have loved before."
"So what's your consciousness doing in a girl on this back water world?" The Doctor wanted to know. Again she smiled.
"This is only a tiny part of my conscience. The rest remains on my planet, where my people await the return of our lost Magi."
"You said that word before. What is a 'Magi'?"
"And I thought Time Lords knew everything," she said with a smile, "She is the leader of my world. Every three generations, a child is born of my line who inherits some of my own power. They are placed into power and the entire planet pays homage to them. Andromeda is the current Magi."
"Andromeda?"
"She thinks it's a bit of a mouthful. Anyway, her parents managed to get her away from the planet in time to save her life. She was only a little thing, barely a few months old. How she came to be on this planet, Matros has already told you."
"What did she need saving from?"
"What do heirs of any true blood line need saving from? People who are unhappy with the system," here the goddess' face pulled back into a snarl, "I hope she gives them the justice they deserve. They have pilfered my planet long enough. It is time for the true Magi to return home."
"She isn't ready." the words were out of his mouth before he could stop them. But on reflection he knew they were true.
"I believe the same. I am glad that you arrived here, Doctor, for I need your help. Take Andromeda with you when you return to the stars. Show her the wonders and dangers of the universe. Teach her justice, control, but above all else, teach her leadership. She must be ready when the time comes to return."
"She's only eighteen. You think she's strong enough to carry the burden of an entire planet on her shoulders?"
"Have you had your eyes open at all tonight?" here she lifted Andy's left arm up and showed the Doctor the forearm. He almost fainted in shock. There was a scar there in the exact shape of the sign for the Devouring Dark, "Relax, it is an exit wound. She is the only person in history to wear this. She should wear it as a badge of honour. And I know that she is the strongest of my children there has ever been. Not many of them could have defeated the Dark."
Once the Doctor had his heart rate under control he suddenly realised,
"You said you were here to protect her. How did…that…get in her in the first place?" Gaia glared at him, then she snapped,
"I did not realise what it truly was until it entered her, and then I could do nothing about it. I have helped her in small ways to combat it throughout the nine years but ultimately it was all her. But mark this Doctor, I shall be on the alert now, and if ever she should need me, all she need do is ask me. Tell her this."
"What else do I tell her?"
"Whatever she wishes to know, of course. Now I must retreat. Please catch her and then take her into the woods to replenish her strength. And in the morning, take her with you when you leave." With those last words, the vibrant green slowly began to fade from Andy's eyes. As he watched her body began to sag. He quickly caught her before she could collapse on the ground like Matros. She seemed to have shrunk to her normal size. She was Andy again. Gaia had gone.
No, not gone, he thought, Retreated. That was the word she used. She's still inside her, but she's a good entity, instead of…the other one. He had learned his lesson. He didn't even try to think its name. With a sigh and an apologetic look at Matros, he hoisted Andy up into his arms and bore her away to the woods.