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The Master had grown something human in the Doctor.
Something utterly mad, but astoundingly beautiful. Feelings the Doctor had only welcomed amongst his human companions. Something that left him burning for his old friend more than any child could. Something that grew into his two hearts, not unlike a weed that festers and quells his own blood.
When his home burned, the Something swallowed the Doctor whole. For one second of eternity, it eased away with Rose Tyler. She’d left her own mark on him, a beautiful blossom in his brain. Before it died with her, and the weeds took root again.
The Something exploded when he’d found the Master again. The Doctor could hardly be himself anymore, his whole being surrounded by the other Time Lord. He could only describe it as a sickness, one due to the spring pollen in the air. The Something’s Sickness was addicting as it was miserable. The Doctor couldn’t be satisfied until his entire body rotted for the Master.
Rot, he did. The Master tortured his friends for a year. Yet, the Doctor found himself begging for his friend’s life. Begging for Kochei’s life. He shouldn’t have, by the universe he shouldn’t have, but he did. The Doctor wasn’t the Doctor anymore, no, no, the Something took his place. He could do nothing but allow it to grow into his shell. The Something had no regard for the Doctor’s friends, it was selfish and cruelly human. It existed purely for the Master.
Until he died. It took one bullet for his life to fall, one bullet and he was holding everything in his arms, one bullet and the Master was dead. The Something died with him. The Doctor? Well, he died too.
Until there was nothing humane about him anymore.