If he had known that by creating a Horcrux half of his soul would end up shoved into the nearest magical object, he would’ve stopped carrying his diary everywhere.
But he was a Slytherin. And a half-blood.
And diaries left unattended in his house were fair game, whether it be for blackmail or… other purposes. If Tom was completely honest, anything and everything could be used as blackmail. He’d gotten ample taste of it his first year at Hogwarts before he’d learned who to cultivate and who to cut. He’d also learned how to protect not only his person but his secrets as well. Which meant his diary, his precious heavily-bespelled diary, went with him wherever he went.
So, of course, his diary was in his bag as he emerged from the Chamber of Secrets, his beloved basilisk beside him, to discover that irritating wretch Myrtle blubbering in the toilets. One whispered hiss and the Mudblood was dead.
It was too good of an opportunity to pass up.
Hastily -- too hastily, in fact -- he whispered the words and made the proper motions to split his soul and bind it to an object of power. Immortality would be his. High on adrenaline after his first kill, he rushed through the ritual. With a final flick of his wand, he felt his soul rip in two and fly out of his body.
He’d expected the piece to enter the basilisk.
He’d forgotten about his diary.
Tom writhed on the floor of the girls’ bathroom in agony. Why hadn’t the texts prepared him for this deep, aching pain? The fracturing of his soul in twain hurt more than any cuff or burn. It hurt more than any injury he’d ever received.
And worse -- as he learned by experimentation throughout the rest of the school year -- the closer the Horcrux was to Tom, the more pain it caused the both of them.
Because the Horcrux was alive.
He hid his diary away buried under layer upon layer of protections. But in Slytherin, the more spells that were on an object, the more enticing it became.
Abraxas was the first to fall under its sway. Tom found the seventh year huddled over the diary, quill in hand, oblivious to the fact that he was sitting unguarded in the middle of the common room. He was so engrossed in whatever the Horcrux was saying, he never even knew what hit him. A quick Stupefy followed by an Obliviate and the Horcrux was once again Tom’s.
What did you do? he wrote that night, pushing past the pain.
What you would do, the Horcrux responded as if it was answer enough.
And it was. Tom knew exactly what he would do. If it were him trapped in that diary, he’d devote every ounce of energy into escaping. He’d drain the life force of any who spilled ink upon his pages. And that included valuable allies. Cultivated allies. Indispensable allies.
Deliberately, very deliberately, he left the diary at the orphanage that summer. A simple Notice-Me-Not and a couple of Muggle Repelling charms and he didn’t have to think of the diary for years.
Until it popped up in the hands of a young Mudblood, a silly little twit raised in that twice-damned orphanage.
The child died swiftly.
But Tom was once again faced with the problem of what to do with the Horcrux.
Through the 50s, 60s, and 70s, he tried various hiding places. A cave. His mother’s old cottage. The Forbidden Forest. But each time, the diary found someone to enchant. Someone to control. It was only through luck and several carefully crafted alarm spells that he kept the horcrux from fully consuming its victims. By this time, Tom was no longer Tom Marvolo Riddle. He was Lord Voldemort. And he had crafted four more Horcruxes. He’d grown more powerful. More notorious.
All of Lord Voldemort’s Horcruxes caused him varying degrees of pain. But none of them pained his as much as the diary. Worse, none of them had the strength of will, the drive to escape, as much as his first.
The diary knew it. Hated it. His teenage master plan for a new, better, world had mutated, changed… lessened. Halfbloods and Mudbloods no longer had any place in his new world order. No longer was he interested in creating a new system of government; instead he and his followers lived for causing terror.
Lord Voldemort was insane. A shade. A shadow of his former, glorious self. And the diary abhorred it. It increased its attempts to escape. Increased its mental attacks. It was no longer content just to wait. It knew it needed to act.
And it did.
The attack was swift. Abraxas’s gossipy silly wife, Anophelina Malfoy nee Skeeter, couldn’t resist pouring her heart, soul, and consciousness into the manipulative journal. The diary roared to life through her even as she died. The battle was epic. Exhausting. Once again, only sheer luck saved Lord Voldemort. A wand mismatch meant the horcrux’s spells were slower to fire, less powerful, harder to cast. A quick Avada Kedavra and the horcrux was once again subdued. Dormant.
But not dead. Not destroyed.
In desperation, with the half-heard prophecy looming, he gave the diary not to his most loyal follower, but to the follower with the strongest willpower. He bade the young father to hide the book most carefully. Keep it away from those who were weak of will or easily influenced. Like a child or the mentally deficient. Or his own parents. Voldemort trusted Lucius would keep it safe -- for the sake of his child, if nothing else.
The diary, the trapped forever youthful Tom Marvolo Riddle, simply waited. Time was on his side. Sometime in the future, when the now Swiss cheese shell of a body least expected it, he’d make his escape.
And then, he would do what he’d always planned on doing. Remaking the wizarding world into his image.