Donnie was in his lab, creating another batch of retromutagen. Potions and vials surrounded the turtle as he swirled a purple liquid around in a beaker, testing for acidity. It was the perfect idea.
This was the theory: If he mixed a certain type of DNA with the existing retromutagen formula, he could control what the user would de-mutate into. Initially, he could turn the user back to their existing form, or change them to their other form - in other words, he could change them into the creature that possessed the other DNA, rather than the original. Following this formula, Splinter would become a rat rather than a man, Rahzaar would become completely canine rather than human, and he, Donatello, would become a teenage boy. It was the perfect idea. He could finally be with April without worry of being outcast and rejected for his appearance. And maybe April would accept his new form more than the old one.
...So where did it all go wrong?
Donnie took a deep breath, filled a syringe with the purple liquid, and injected it into his arm, watching as the needle penetrated flesh and muscle, wincing slightly. He put the now empty syringe back on his desk. For a few silent moments, nothing happened. Donnie was beginning to wonder if he'd done something wrong when it happened.
A searing, burning pain broke out all over his body all over at once. He was paralysed. Donnie fell to the floor, crying out in pain, short spasms jerking through his body, forcing him to curl up in agony, groaning loudly. It was unbearable. Donnie's mind was racing. What was happening?!
Leo was watching the latest episode of Space Heroes when a pained cry cut through the air, breaking his focus on the screen. Leo recognised the voice. It was Donnie. "Donnie!" Leo was up instantly, running into the lab, wondering what on earth had happened. He looked around. The lab was chaos. Liquid was dripping from Donnie's desk and on to the floor, there was broken glass everywhere, and there was electrical wiring, ripped from its sockets, tangled around various bits of machinery. Donnie was nowhere to be seen. "Donnie?" Leo called out, half in worry, half in confusion.
A low, rumbling growl filled the lab, reverberating through the air, making Leo edgy and nervous. What was that? A pair of burning yellow eyes flashed open from the darkness of the ceiling above Leo's head. The blue-masked ninja didn't even hear it coming. One minute, he was standing there, and the next minute he was lying flat out on the floor with... well, with this thing on top of him.
The creature's skin was pale green and was marked with light mottling. It had long arms and legs, ending in wicked claws the length of throwing knives. it stared Leo down with those burning yellow eyes, it's pupils dark, foreboding slits which seemed to promise death to anyone who crossed its path. On its back, a gathering of jagged, rock-hard spikes running along its spine seemed to indicate a shell. It screamed into Leo's face, showing an impressive set of shark-like teeth, filed to needle-like points, serrated around the edges. A tail, long and ending in a crooked curve, jutted from its rear, lashing from side to side.
The creatures claws cut into Leo's shoulders and chest like a knife through butter, the red of his blood staining its hands. It snarled, pinning him to the floor, staring into his eyes. That was when Leo's noticed it. A piece of tattered purple fabric tangled around the creature's neck. It couldn't be. "...Donnie..." Leo's eyes widened. The creature seemed to recognise the name, and for a moment, it loosened it's grip, a mixture of anger and surprise etched into its features.
Leo took his chance. Pulling his legs free, he gathered his strength and kicked up at the creature, throwing it off forcefully. It was hurled towards the far wall, screeching its rage at Leo. It hit the wall back-first, slamming into the cold stone, before falling to the floor, dropping out of sight behind the Shellraiser. There was another screech - this time of pain - then a softer moan, and finally silence. Leo stood slowly, his legs trembling, his head too pumped full of adrenaline to notice the blood running from deep slash marks on his shoulders and thighs.
A few moments passed. Then, a small, scared voice called out, the sound barely audible. "...L-Leo?" Just before he heard his name, Leo looked down, realising the blood loss to his system. Slowly, almost painfully, he crumpled to the floor, out cold.
Donnie walked out from behind the Shellraiser, shaking badly himself. He felt a deep, aching pain as if he'd just woken up after going on a 30 mile hike the day before. He stumbled, rather than ran, over the Leo as soon as he saw his brother lying on the floor, a pool of his own blood collecting around him. "Leo!" Donnie was shocked. "Speak to me, Leo!" Nothing. His hands felt wet. He looked down.
Donatello couldn't have possibly comprehended what had happened. It could have been a thousand things. But looking down at his own hands, stained red and dripping with Leo's blood, he knew. It wasn't possible, but he knew.
"...I did this."