-1I'm leaving a portion of the story out because it's not exactly gonna fit under a G rating. I know you'll figure out what happened, though.
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Pebbles rained down the cliff face upon the Autobots who stood as one and tried to think of what to do. "Go up the cliff?" That was shot down. "No, the rock's not strong enough to hold us." Another idea was put forth. "Go around?" That was shot down as well. "We wouldn't get there in time." Frustration took hold. "Well, there has to be something!" Not a one of them was against trying to save Megatron if they could. Former enemy or not, he had earned his place among them and become a comrade, no, a friend. Ratchet was getting ready for the direst of emergencies and praying with all his might to whatever god was listening that it would be damage he would see; just damage and not death. His hope was already dying, and it faded more and more with each savage grunt and pained cry.
Sarah, for her part, was half past hysterical as she tried again and again to use her power, to take control even despite the fact that she was out of range. The only thoughts running through her mind was that he was up there, he was alone up there, outnumbered and possibly incapacitated, judging from Starscream's words moments before… This whole war was her fault; she was supposed to have all this power, the power to give life and rejuvenate worlds, but what good was it doing her now? She was supposed to have power over life and death, so why was she powerless to save the life that mattered most to her? There had to be a way to fight, a facet to her power that she hadn't considered yet…
Anger swelled inside her, hatred of those Decepticons; oh, how she longed to be able to fly up there and make them pay for everything! She'd never been this angry before in all her life. In her experience rage burned as a searing flame, but never before had it burned quite like this! Before it had only burned in her chest, but now it surged throughout her whole body. Fire was rushing through her, saturating every fiber of her being, filling her, energizing her, empowering her, becoming her… and that fire was blue.
A high, piercing scream drew Optimus's attention away from the cliff where he knew his brother now struggled to survive and made him focus on the torrent of wicked blue fire erupting from Sarah where she stood. The scream had come from her; not a cry of pain but a wordless expression of pure black rage with an undertone of sheer wild elation. All around him the other Autobots were staring as well, wondering if the All Spark's power had somehow turned on its bearer. The fire raged stronger, rising higher, burning brighter, until they could see nothing save a pillar of roiling, churning blue…
The All Spark's energy faded away, but what stood there… wasn't Sarah. Huddled on its knees, panting and badly shaken, was a being that looked like a slightly smaller version of Megatron; that same spiky armor, those spidery claws… Rather than being silver, it was a shade of gray so dark as to be almost black, glittering in the light as the blue died away. The black was far from perfect, as delicate traceries of that same bright blue trimmed every spike and every curve. The eyes were different, the same color as the highlights that marked its body; blue lightning instead of crimson fire. As they watched, a few seconds elongating into an eternity, the dark being stood, revealing vaguely feminine curves. It was then that they knew. This was Sarah, and she had yet to realize what she had become, as if her mind was on autopilot, some unknown force guiding her. A small rain of pebbles drew her attention upward and she abruptly looked up, newly formed fangs bared in a look of hate-filled fury that reminded Optimus of how his brother had been during the war. Crouching down, she leapt straight up with a sharp, brutal grace she had never shown in her human form… and instantly pulled off her first transformation, becoming a darker version of that infamous alien jet and rocketing straight upward on a pillar of purple fire.
The scream wasn't lost on the combatants above. Ramjet and Dirge were working together to both help support Megatron and to restrain him so he couldn't attack the evilly smirking Starscream who stood before him, taunting him in his moment of weakness. Starscream, for his part, was enjoying each and every millisecond of what was going on. He'd already more than humiliated Megatron, already more than hurt him… He'd made sure that his former superior had paid dearly for denying him his desires and being in his way, but still that wasn't nearly enough for him. He wanted to hurt him more, and hearing that scream made him realize how he could do that… "Do you hear that? The girl screams for you." His smirk only grew as he saw his former leader's eyes narrow at the mention of the All Spark girl. Evidently he'd touched a nerve. "She's next, you know. After I'm done with you, I'll see if she can scream any louder!" Snarling wordlessly, Megatron tried to lunge for Starscream's throat, practically dragging both Dirge and Ramjet along with him. "You leave her out of this!" The two Decepticons just barely managed to hold him back as he fought with all his remaining strength to break free and strangle his former second in command. Taking a step back at his former commander's sudden burst of strength, Starscream knew he had gotten to him, and this pleased him greatly. "As a matter of fact, I'm not sure who to kill first. You… or her."
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Now she can transform too, it seems. It must be a self-defense adaptation caused by the All Spark.