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Halfway through the train ride back to London and the four in the last compartment of the last car were reading various books and magazines, Neville suddenly said, "The one thing that no one ever figured out was who the 'Other Heir' was. He fired off a few messages about Salazar Slytherin and then disappeared."

Hermione gave one of her patented huffs, and said, "There was no way of telling."

Luna smiled serenely and said, "No, there were a few clues, Hermione, just none of them direct. There was another known parselmouth in the school, who had a juvenile basilisk as a familiar, and who was reading a thousand year old green leather bound journal like a man possessed during the two months that the attacks were occurring."

Then Neville and Hermione stared at Harry as Luna went back to reading the latest issue of the Quibbler. He was shamelessly avoiding looking at anyone at all by burying his face further in the book he was reading.

Neville shook his head and said, "Harry, you are utterly incapable of lying to anyone who you consider a friend. Even if you say nothing at all."

Giving a wry smile as he showed his face, he reached down into his bag and pulled out a green journal. "The personal diary of my ancestor, Salazar Slytherin. There are you happy? The really frustrating thing is I read this for every spare moment I had during those two months, and I finally found the sections on the Chamber, on the first sodding day of the new year, a week after I went up against the basilisk"

http://www.fanfiction.net/s/8253087/8/