Author's note – I have no idea when this could have been set, after Sirius' death but any time between then and the big battle. It's heavily inspired by the end of the OoTP movie when Voldemort possesses Harry but I put a slight different spin on it? I don't know. All characters owned by JK Rowling.
A hand curled around his in the gloom of the garden. He'd been expecting it but it still gave him a fright. Harry glanced around hurriedly waiting on the voice. The voice that would enter his head. He tried to remember what Dumbledore had instructed him to do. Close your mind; only show him parts. The advice seemed indistinct now. How was he to close his mind to the most powerful wizard alive?
He shook his head, again turning back to the task of blocking out his memories, his thoughts, from the Dark Lord. Tucking away memories of his youth, ideas and plans from the Order, conversations with his friends. Instead he chanted the information he needed Voldemort to know: he's at Hogwarts.
Harry. It came to him in a hushed whisper, the voice dulling the meagre light that had shone through the darkening clouds. With a shock, Harry felt all the breath leave his body at once and he collapsed, gasping, clutching at his throat. He's at Hogwarts. He thought again but Voldemort didn't retract. Instead, Harry felt his presence seeping further and further into his mind. Felt it pressing up against his closed off thoughts, attempting to tear them from their hiding places. He's at Hogwarts, he thought desperately, unable to breathe. His vision started to blacken, stars shooting across his eyelids.
Where are you? There was almost a sing song note to the question, like Voldemort was playing Hide and Seek. Harry ignored it, slamming back the thoughts that would reveal himself to the Dark Lord. He's, he gasped in his mind, at Hogwarts. Who. But Harry didn't reply. He knew that Voldemort would have assumed immediately. He forced himself not to think the answer for fear of memories being uprooted. Suddenly, Voldemort entered his mind fully in a rush of pain. Harry would have cried out if he had a voice to cry out with. Instead, he curled up in the floor, unable to see but for a mist of black and red spots, unable to think other than the pure panic that enveloped his body.
Harry. The voice was closer now, echoing and reverberating in his ears, his head. Weak. The single syllable made him convulse, his body squirming uselessly in the yellowing grass. You always were weak. And his voice was like agony, shards of glass drilling into his skin, making him bleed blood that came out clear. He screamed then, his blind panic overcoming Voldemort's control for a brief moment, before he pushed Harry aside once more. Even as a baby. The words ripped out the memory before Harry could react, the scream, the flash of green light. His mother. His body convulsed violently as Harry and Voldemort fought for control. "You're the weak one," Harry choked, no longer able to speak in his mind. Lies! But it was as if a dam had opened and memories were pouring out, overwhelming both of them. Sirius falling through the veil, Ron screaming at him, the bars on his window at home. His parents screaming. His parents, Harry honed in on them, on the grief they had left, on the warmth they had given him. On their humanity. He forced the images to display his happiest thoughts, his friends laughing gleefully in the common room, the feeling of the air rushing through his hair when he flew a broom, Sirius wrapping him in his arms. Stop this. Voldemort's voice came through, tinged with panic. Stop! The images slowed but they didn't stop, flashing across his vision, giving him the strength to respond; "Get out of my head," he told him forcefully. Voldemort flinched in his mind, but he didn't leave. "Out!" He yelled it, screamed it. Out! He repeated in his head. And Voldemort did. With a horrific spasm in Harry's body, the Dark Lord seeped from his mind with an extended hiss that Harry was sure he had repeated through his lips.
His surroundings started to materialise around him once more, the sound of a bird tweeting in the background, the feeling of grass digging into his back, a hand on his shoulder. With a start he opened his eyes to find an anxious face looking down at him.
"Ginny," he noted, before promptly passing out.