Written for the prompt the writing on the wall at gen prompt bingo
The words on the wall seemed to hold Harry captive, he knew somehow that they were for him, although he was unsure what he had done to become the heir's enemy. His mind cast back to the odd dream he had after his sorting in first year with Quirrell and his denying a place in Slytherin. Was not being a member of Slytherin house when he was supposed to be something that would have made him an enemy of the heir. He wondered who the Heir of Slytherin was, whether he had done something to offend them personally like he had Malfoy.
Malfoy's comment about mudbloods being next made Harry wonder if that was why he felt he was one of the heirs' enemies, his mothers and his blood wasn't pure. That didn't seem right though, the writing on the wall seemed to him as if it was directed straight at him, it was more personal than blood status.
Harry became obsessed with finding out who the heir was, he wanted to know why he felt he was their personal enemy. They brewed polyjuice and snuck into the Slytherin dorms, but Malfoy had no idea who it was. Harry had been writing to Tom who had shown him his capture of Hagrid, but that didn't seem to fit. Hagrid had no reason to think of Harry as an enemy, or to attack students.
They took Hagrid's advice and followed the spiders that fled the castle and almost ended up being eaten by acromantula, but they were still no closer to finding the heir of Slytherin. When Ginny was taken they knew where the chamber was and what the monster was, the only thing they didn't know was who the heir was, although that became somewhat less important than rescuing Ginny.
Harry had been glad in a way to find that Voldemort was the heir, even if he did point out a lot of worrying similarities between the two of them. It meant that there was a personal reason for the hatred of the heir, he had known that the aura of that message was directed at him. At least this meant that he only had the one real enemy still, he didn't know if he would be able to cope with multiple enemies, it was bad enough that Voldemort had all his followers at his disposal.
The mystery behind the writing on the wall was solved and Harry felt a bit safer knowing that Voldemort's return had been thwarted once more, and that Dobby was now a free elf and had promised to never try to save Harry's life again.