It's two books and a snippet of information that help Draco figure it out.

He'd never considered himself extraordinarily intelligent, not like Granger, but he is both extraordinary and intelligent. It's enough.

The first book he discovered the evening before Harry Potter forcefully acquired the origins of the now iconic spell.

'Secrets of the Darkest Arts.'

Draco wasn't at all surprised at first to find the book hidden amongst he and Tom's shared bookshelf, at first. It was obvious that Tom did a bit more than dabbling in the Dark arts from the other books he'd found— and Tom's mention of the book being created by Dark magic. What was surprising, however, was the book's solutions to obtain immortality.

Draco was fairly certain that Riddle was immortal— he'd been here since he was sixteen and hadn't aged a day. Draco guessed that however he had managed such a feat might be contained within this book.

His curiosity is why Draco stumbled upon Horocruxes.

If Tom's insistence that he was merely a manifestation of memories proved true, then Draco would not even bat an eye at the interesting text. But Draco knew Tom was lying, in that heart he'd buried he knew it. That Tom was more.

Perhaps, he was even a soul. Or half a soul.

It all fit together— why Tom felt the need to lie about it to Harry, because the process of creating a Horocrux involved murder. It explained why Tom expected his and Draco's experinces in the diary to be the same.

Tom Marvolo Riddle was a Horocrux.

Draco stared in shock— fear— at the boy sitting across from him. He appeared so innocent, so caring for Harry, just a characteristic teenager. But he was a murderer. Harry deserved to know, Draco then decided. As soon as the diary glowed again, he'd tell Ha— Potter, he'd tell Potter.

"Is there something wrong, Malfoy?" Tom asked, interrupting his unintentional glaring.

"Erm," Draco exhaled lightly, "No."

"Really?" Tom pushes. "Because it seems you've realized something. Or is there something on my face, hm, Malfoy?"

He stiffened. "I have no idea what you're talking about."

"I should've assumed you'd figure it out," Tom hummed, "You always did strike me as the intuitive type— and a pain in my ass."

"Figure what ou—"

"Secrets of the Darkest Arts," Tom interrupted. "It's a lovely read, isn't it?"

Draco held his breath. Fuck.

"You can be content with your knowledge, but you mustn't share it to my dearest Harry."

"And why not?" Draco snapped.

"Because I'll tell him your dirty little secret as well," Tom grinned.

"I-I don't have—"

"Oh, but you do," Tom cackled. "It's so obvious, even I was able to put it together. It would be such a shame if Harry Potter knew that Draco Malfoy had a crush on him."

Draco said nothing. It'd ruin him, his reputation, his relationship with his father, if the news got out. There's a war, a battle, going on in his head, because Tom's in no way incorrect about his assumptions— he never thought himself this obvious and yet— but Harry does deserve to know.

He'd keep his mouth shut. For now. (He always has been a coward.) His shoulders slumped with resignation and Tom's smile widened.

Tom opened his mouth to say something but the glow of the diary interrupted him. Harry was writing. They flipped open their books, Tom locking eyes with him as he did, almost as if daring him to say something. (He doesn't.)

'Voldemort told the spell to Goyle,' Harry wrote. 'That's how Ron got it from hit.'

Draco frowned. At both the use of He-Who-Shall-Not-Be-Named's name and the simple fact that something didn't (fit) make sense here.

Draco dismounted from his chair, leaving his dairy opened on the table with Tom, who he abandoned and gave the burden of taking to Harry about this to. He mulled over the strange tidbit of information as he scoured the bookshelf. It was so suspicious, the Horocrux, the sudden connection with the Dark Lord—

He stopped his train of thought when his eyes latched onto an old scrapbook. He recognized it, he thinks, if only vaguely. He pulled it off the shelf, grateful for the distraction.

He'd read it when he was four— maybe five. It was a scrap book of his grandfather's, Abraxas's. He'd found it hidden in his father's study. He wasn't supposed to be reading it, so it got confiscated as soon as Lucius saw, but he'd already flipped through it all by then. Lucius did give an explanation for the book, if a rather cryptic one:

"It is a part of the past we are meant to be burying, Draco. Let the dead stay dead."

He'd forgotten all about it, until now. Draco knelt to the floor, not wanting to move closer to Riddle, and flipped through the pages. It was his grandfather and friends. He recognized them, if only by what their children and grandchildren looked like.

There was Abraxas... Lestrange... Rosier.... Black. A few he didn't recognize. All future Death Eaters, Draco realized with a frown. He stopped flipping through the pages abruptly.

That's... that's Riddle.

He looked up at Tom, then back at the page and the results stayed the same. Draco could recognize most everyone else on the page, and Lucius had mentioned in passing that Abraxas had been friends with the Dark Lord in school—

Draco's breath hitched. Tom Marvolo Riddle is Voldemort.

It made complete sense now, why Voldemort knew about the spell that sent one into the dairy. He'd likely planted the diary or something similar, had created it as well, in an atempt to be immortal and then, after his large fall from power, sent in motion a plan to send someone else into the diary as well... but why? What did he gain?

He'd think about that later, but all that mattered currently was the fact that Harry Potter was unknowingly befriended with Volde-fucking-mort. Harry needed to know— no matter the cost. Riddle was dangerous. He'd likely tell Harry that Malfoy liked him, that darn Riddle would, but it'd have to be worth it. And Harry would probably give the diary to Dumbledore or the Ministry...

And the diary might get destroyed with Draco intact. That's... Draco took a deep breath. That's fine. The Dark Lord needed to be struck down before he could become even more of a threat. It's for the Greater Good.

Draco slid the scrapbook back onto the shelf, walking back over to he and Tom's shared table with what he hoped appeared to be nonchalantness, and sat in front of his diary. A frown appeared on his face when he realized that Harry was gone, off back to class, and it might be hours until he showed up to write again.

He could almost growl in frustration, something entirely unbecoming of a Malfoy, but held himself back. He could wait. He is no stranger to playing the long haul and this would hardly be long.

As long as Tom Riddle remained oblivious to the turmoil of emotions and plotting going through his mind, all would be well. (As well as it could be.)