And why didn't he tell anyone?!?!?!?!????
I really can't figure out the point of his existence either. Having to go to the Ministry and take the locket from Umbridge doesn't lead to any clues about other Horcruxes and also doesn't lead to them being caught. The story would be essentially the same if it had been a real Horcrux that Harry had found at the end of #6.
As I recall, there's no explanation in the books about how Regulus found out about the horcruxes. Regulus turned against Voldy then apparently found out about at least one horcrux, and took it, substituting another locket.
The trio destroy the locket at the ministry because they need to find and destroy all horcruxes.
I take your point about the fake locket with the note from RAB. Just another plot twist that Rowling added.
The movies cut out a lot of back story, including Voldemort and other death eaters/pure blood lack of respect for not only muggle but of other magical creatures - elves, centaurs, werewolf, etc. Regulus Black's elf plays into this story of the locket, and the disdain Vol shows towards the elf.
Second, the scenes at the ministry shows how far so soon the ministry had fallen, and showed what Harry and gang were really up again. Enemy No. 1. The people who were on his side didn't have the strength or courage to stand up for him. Plus, they were getting comfortable at the house and they were forced to start traveling which led to them learning they were in over their heads, but also, forcing them to try. They spent months at the house planning to break into the ministry. Months. Had they been able to return, who knows how long finding the horcruxes would have taken.
In the books Voldemort goes to Regulus and asks for the use of his house elf. Voldemort then used Kreacher to test and place the horcrux locket. Kreacher came home to Regulus as Regulus had commanded of him despite the potion and the inferious at the black lake. Regulus got the whole story about the locket and the potion and the lake and that Voldemort left his loved and loyal house elf for dad. Regulus, being a death eater and deep into dark magic, either put the clues from the story Kreacher told him together or heard something in his capacity as a death eater or both. Regulus then writes the note, puts it in another locket, takes Kreacher back to the lake, sacrifices himself instead of the elf, commanding Kreacher to switch lockets and destroy Voldemort's locket, before dying by the inferi.
shareHe found out because Voldemort used kreacher to test the defenses around the horcrox
shareThe fact that the first locket was a fake adds to the despair and potential pointlessness of the monumental task they face. Apparently to the characters, Dumbledore died for no reason at all, and he died having made a mistake - these shake the faith of the protagonists.
shareWhich is brilliant writing, and why I love these books (: Well said, I might add!
shareI actually enjoyed this plot twist. The only part I had a problem with was the plot hole about Regulus's death. In OotP Sirius tells Harry that Regulus had been killed either by Voldemort or the Death Eaters implying that they were aware of Regulus's treachery and Lupin says the same thing in HBP. In DH though he is said to have been killed by Inferi and that neither Voldemort or anyone other than Kreacher were aware. A lot of things didn't make sense here.
Regulus comes from a family who were well-known practitioners of the Dark Arts. Although information on Horcruxes were banned at Hogwarts, Dumbledore had no control on information regarding them at home. It's not completely left field to imagine that the Black family may have possessed books on the matter.
Regulus had heard Voldemort dropping hints about his immortality, put two and two together and was disgusted by what Voldemort was doing. He, however, did not realized that Voldemort had more than one as there is no record of any wizard in history creating more than one as to simply make one was considered the most extreme kind of magic ever performed.
He vowed to destroy the Horcrux (the only one he thought existed) himself, knowing that he was signing his own death sentence when Voldemort found out. His death, however, occurred in attempting to capture the Horcrux so he passed the task on to Kreacher.
He did not tell anybody else so not to put his family at risk, suspecting that Voldemort would come after them if they knew his secret. Sirius believed that Regulus had tried to defect out of cowardice and was killed on Voldemort's orders but this was believed only - Regulus simply disappeared one day and Sirius only assumed that this is what happened to him.
I don't know if Voldemort ever attempted to find him (if only to kill him for apparently defecting) but he probably had bigger fish to fry at the time than a missing teenager.
Ironically, Regulus died a hero and was even willing to make himself look like a coward in death to protect his family and destroy Voldemort. I hope Harry made clear to the world what Regulus actually did in the war and he got the credit he deserved.
I expect that Voldemort probably sent some Death Eaters after him once he realized that Regulus was missing like what he did with Karkaroff, but I agree that he probably wasn't high on his priority list. Regulus went missing only a year or two before Voldemort's downfall when the battle between Death Eaters and the Order was getting more and more heated.
You know what I just thought? Maybe the tapestry automatically recorded his death and his family just assumed that he was killed over the course of the war and no body was found. Perhaps Bellatrix or Narcissa simply reported this to Voldemort who assumed he got killed by aurors. My only other question is why Sirius would've assumed Death Eaters rather than aurors killed him, but maybe the Order somehow knew (or at least suspected) that Regulus was becoming disillusioned to the Death Eater lifestyle. Just a thought.
Ironically, Regulus was likely one of the Inferi that tried to stop Harry and Dumbledore from taking the fake Horcrux.
There were cases in the war of people simply disappearing and them being assumed dead. This is likely what happened to Regulus. Voldemort either thought that he had defected (and was too busy to attempt more than a cursory effort at finding him) or, as you say, that he was killed by the Aurors or, more likely, the Order.
Voldemort had probably completely forgotten about him by the time he rose again and even if he hadn't before, assumed he was dead then as he had not been seen or heard of for such a long time.