It was true, Regulus had indeed been intrigued by the Dark Arts, and the promises of power that Voldemort offered had seemed tempting at the time. But as the young wizard had witnessed firsthand the methods employed by the Dark Lord, he had started having second thoughts. He still considered having been specifically chosen to volunteer his elf an honor; but he would have liked it if Voldemort had been a little more forthcoming as to why he required the services of an elf. Nevertheless, he had ordered the elf to complete whatever task had been set him, and return home.
When Kreacher had indeed returned, on the brink of death; something had snapped in Regulus. Seeing the elf covering against the wall, eyes bloodshot, clutching at his throat, all the while no sound escaping him; brought back all the frustration that he had suppressed six years ago.
In his mind flashed a single painful image of his brother bleeding helplessly in this very house, on a night that was eerily similar to this one. He was older, better prepared to deal with something like this now. The house elf would not die on him. This was the moment, Regulus later realized, he had effectively stopped working for Voldemort.
He had nursed the elf back to life for weeks; in a small, dark, storage space opposite to the pantry. The house-elf himself was under orders not to show himself to anyone outside the family, for Regulus wanted no news of Kreacher's continued survival making the rounds.
The elf had simply refused to talk for the first fortnight, only whimpering painfully when Regulus had attempted communication. On a particularly difficult night, Regulus had reached for the Butterbeer to ease the elf's hysteria. Three bottles later, the elf had started talking; his story ripe with detail, racked with huge sobs and numerous hiccoughs, ultimately quivering until he had fallen into a deep slumber.
Regulus was stunned into disbelief. He had been expecting a different story, one that involved perhaps the development of a new Unforgivable Curse or some such endeavour, given the state Kreacher had been in when he had returned. But this was uncharted territory. It seemed like Kreacher had described the lost locket of Salazaar Slytherin. Regulus had heard rumours of its continued existence, and had also heard a whiff of foul play involving the last known owner.
"Oh, Sirius," he sighed out loud. His brother, with his decided recklessness and staunch anti-Pure Blood policies would be his best ally if he was going to take on Voldemort singlehandedly. But he knew this endeavour would lead to certain death, and he could not knowingly lead his brother down that path. The less people that knew about this, the better. This was his battle, he would fight it alone.
He decided to start with the locket itself. As a Slytherin, he had heard about the lost lineage of Salazar Slytherin. He traced the family back to a Marvolo Gaunt, who had died after a brief stint in Azkaban. Borgin and Burke's had been rumoured to have acquired the locket around the time of Marvolo's death. Regulus had heard tales of Burke's dealings to go poking around in the exact circumstances involved. Shortly afterwards, the locket was sold to a Smith family.
The locket was found missing when an inventory of all the belongings of one Hepzibah Smith had been conducted, quite some time after her unfortunate death. The circumstances of her death had been quite sensational, her house elf had been implicated in some kind of a mix up involving a poison.
The locket seemed to leave behind a trail of mysterious deaths that seemed to have nothing to do with Voldemort. Regulus knew enough about Voldemort's undertakings to detect the murky, suspicious circumstances under which The Dark Lord did some of his best work.
It took him all of three months to figure it out. His search took him to the most ill-advised corners of the Wizarding World, he faced horrors a normal teenager would have run from, he read the most gruesome books on ancient dark and evil magic. As he continued on his search, the words 'experiments in conquering death', and 'path to immortality' that Voldemort had used at Death Eater gatherings began to make sense.
A horcrux. Voldemort had created a horcrux; that was what made him invincible. If that horcrux was destroyed, he could be killed, just like any other man. One simple incantation - Avada Kedavra, and all would be right with the world again.
But something still nagged at him in the corner of his consciousness. Another ludicrous, fantastic tale about the fourth Founder of Hogwarts. The tale of the Chamber of Secrets. If the details of the myth were to be trusted, the Heir of Slytherin could unleash a monster in the castle; and purge all those who, in Slytherin's view, were unworthy to study magic.
If Voldemort truly was the Heir of Slytherin, why hadn't he cleansed the castle of Muggle borns? Surely, The Dark Lord would have had to attend Hogwarts, just like the rest of the wizarding world. And if he had, why hadn't he unleashed Slytherin's dormant monster on the school yet?
Some well placed memory charms on a few unsuspecting elderly wizards in Hogsmeade revealed that the last time the Chamber was supposedly open, a girl had died. This was all the information he needed. He knew Voldemort had killed the girl and created a horcrux. He vowed to avenge her death, and countless others as well.
Was Voldemort, truly, the Heir of Slytherin? If he was, he would be related to the Gaunts. But there was no record of the Gaunts ever attending Hogwarts. This could have been a dead end, but the Gaunt family tree had one person he had not yet accounted for. The daughter - Merope Gaunt.
For all he knew, she had died as soon as her father and brother were sentenced to Azkaban. On a whim, Regulus decided to read up on the circumstances of their arrest, in case it turned up any information about Merope Gaunt. But no amount of research yielded anything worthwhile.
He had come so close, yet, he was missing something. If Voldemort truly was the Heir of Slytherin, it would explain why he went after the locket. But why not advertise i's presence? Why not declare to the Wizarding World that he was the Heir of Slytherin? Why keep it a secret?