Starts in 2x09 "Katerina" when Katherine offers Elena her blood. Writing this over a season later, but the idea just came to me. :) So what would have happened if Elena had accepted it?
"There's another way out." Katherine says as she cuts a place on her arm.
Elena watches the blood trickle out and forces herself to turn her head. "I can't."
"Why not?" Katherine studies her face. "Aren't you going to become a vampire eventually anyway?"
"No. Maybe. I don't know." Elena huffs. She always feels frustrated around Katherine, something about her just brings a hidden quality out in her. Something that most people don't get to see.
"Come on Elena." Katherine smirks. "Last chance."
Elena looks back at Katherine's arm, and on impulse reaches down to grab it. She quickly pulls it to her mouth before she can change her mind.
A sound comes from her throat as the sweet taste of Katherine's blood hits her tongue. What the hell is wrong with me?
"Hmm." She can almost hear the smile in Katherine's voice. "You're better than I thought you were."
Katherine pulls her arm back and Elena carefully wipes her mouth. What had she just done?
She watched as the wound on Katherine's arm healed back up. "Ready for the next step?"
"I don't know." Elena sighed.
"Well my blood is only going to stay in your system for so long, and I'm not offering up any more."
"I need to talk to Stefan and Damon first."
"Why? It's not like you'll actually be dead."
"What if I can't complete the transition?"
"Then you'll be dead dead. But you're a Petrova, I think you can do it." Katherine pushed the cup back toward her and this time after Elena filled it with more blood she pushed it back with her hand. She had a feeling that Katherine didn't plan on actually hurting her. "And besides," Katherine continued after she drank the blood. "Damon and Stefan wouldn't let you go through with it anyway. So why don't you make your own decisions for once?"
"I do make my own decisions."
"Want to try that again?"
Elena just sighed. Katherine gestured for her to come toward her. But Elena didn't move.
Katherine smiled. "Come on Elena, why don't you be the one to surprise everyone for once?"
Elena couldn't help it as she rose to her feet and started to move toward Katherine. She had to give her one thing, she was damn convincing. And this was probably how she'd kept herself alive all these years.
Elena let out a breath as both of her feet crossed the entrance to the tomb. Katherine could easily kill her now if she wanted to. But she didn't.
She just sat there smiling up at her. "Good girl."
Elena rolled her eyes. "Are you going to do this or not?"
Katherine stood up. "Of course." She ran her finger across Elena's collarbone and Elena involuntarily shivered under her touch. "I really like this side of you." Katherine whispered into her ear. "You should show it more often."
"It seems to only come out around you."
"So I bring out the best in you."
"I'd say it's more like the worst."
Katherine smiled. "We'll see about that." And in a matter of seconds Elena felt Katherine's hands on her neck and then everything went black.
Elena woke to someone stroking her hair. It felt familiar and she assumed that someone was Stefan, so she snuggled closer to them. She brought her arm up to touch Stefan's leg, but she felt that it was much more feminine and similar to her own and that was when what had just happened hit her like a train.
She quickly jumped up. "Katherine."
Katherine just tilted her head. "Forget what happened for a minute?" Elena nodded. "That happens sometimes. Drink some of the blood you brought, you'll feel much better. And you'll find out if Damon or Stefan ever compelled you."
Elena looked at the blood bag and than back at Katherine. She wanted it so badly she could feel it burning in her throat, but her feet wouldn't move.
"Don't back out now Elena. I really don't want to face the wrath of the Salvatores for killing their girlfriend."
Elena leaned against the wall. She could fight this off for a few minutes, or at least try. "Why are you doing this?"
Katherine sighed. "Do you want the truth or the answer that will satisfy you?"
"I want Klaus to get here and see that there's not a doppelgänger that he can use for his stupid ritual and that there is never going to be another one." She stood and moved closer to Elena. "And as much as I hate to admit it, I actually care about you Elena. I don't want to see you die over an altar for that asshole's benefit."
Elena couldn't help but smile a little bit. "What was the answer that would satisfy me?"
"That we could defeat Klaus."
"But we can't?"
"Not unless we can come up with a damn good plan."
"I think between the two of us we should be able to come up with something."
Katherine seemed surprised by this, but she quickly regained composure. "Then you're going to need some of that." She gestured toward the blood bag.
Elena nodded and walked back to get it. It was now or never. If she did this she could help protect everyone, Klaus would have to kill her first before she let anyone die for him. She knew she would regret this later, but she brought it to her lips anyway.
She sucked every drop of blood out of the bag until Katherine told her to stop. "We don't want you to have a bit of a blood problem like Stefan." Elena nodded knowingly and walked back into the tomb.
"So what do we do now?"
"Elena!" She heard Stefan's voice.
"Oh no." She wasn't ready to deal with this yet. "What do I do?" She looked back to Katherine.
Katherine shrugged. "This ones your call."
Elena shot her a look before stepping back outside of the tomb. She was going to try to conceal this the best she could for now.
Stefan rushed in and stopped when he saw her. "What are you doing here?" He gave Katherine a dark look.
"I thought she could help, maybe tell me something about Klaus." Elena crossed her arms across her chest.
"All she will do is lie and manipulate you Elena." He shook his head. "You should've told me."
"I don't have to Stefan. This was my decision." She saw Katherine smirking out of the corner of her eye.
Stefan nodded. "You're right. I'm sorry. I was just worried." He came over to her and pulled her into his arms. But he pulled back after a second. "Katherine?" His eyes searched her face.
"What?" Elena stared back at him.
"It's just that I couldn't hear your-I thought there was something different about you."
"Don't worry Stefan, she's still the same boring Elena Gilbert." Katherine spoke up. She was actually covering for me. Maybe we actually could work together.
Stefan seemed to lose his suspicions, but Elena rolled her eyes for good measure.
"Now can you take your lovers spat somewhere else? Some of us have better things to do, like watch a mouse run back and forth." Elena had to stop herself from laughing.
Stefan took Elena's hand and they started to leave. Elena knew she was going to hear him fuss about this all the way home though. Elena looked back over her shoulder and Katherine winked. "Kill Elijah" She mouthed. Elena barely nodded and realized that maybe she actually cared about Katherine a little bit too.
Do you guys think I should continue or leave it as a one-shot? I'm not really sure. But thanks for reading, and reviews are always greatly appreciated! :)