Alternative reading: what if the locket was a decoy?
So I was thinking about the locket Horcrux and I suddenly thought of a really interesting alternative, against-the grain-reading. I think an argument could be made that it was never a Horcrux and that Tom intended Kreacher to escape and the whole thing was a trap to catch a spy and undermine the Order that went slightly sideways. I'm not saying this is what I necessarily actually headcanon, but I think it's a really cool concept. Let me explain.
It's really interesting to me that according to Kreacher's account, the locket wasn't placed in the cave till shortly before Voldemort was destroyed the first time. In Kreacher's story, Voldemort didn't use him to test the defenses. He has the locket with him and places it in the basin after Kreacher drinks the potion. So...where was it before?
Either he moved it from another location (which has some concerning implications of its own since that would mean there could theoretically be some other undiscovered Horcrux in the old location) or he didn't acquire it till much later than suggested in canon OR it was a setup.
Tom may have suspected that Regulus was a spy. Regulus had a brother in the Order after all. And he also may have already started having doubts about Tom and The Death Eaters (and Tom is a powerful Legilimens and may have detected this). Furthermore, depending on exactly when Lily & James went into hiding relative to Regulus's death, Snape may have already been spying for the Order so Tom may have been made further suspicious by the fact that information about some of his plans seemed to be leaking. He may have devised the locket as a test and a trap.
Maybe it wasn't a Horcrux at all, but rather an object designed to corrupt people and sow discord in a group and Tom's plan was that Regulus would recover it and bring it back to the Order where it would cause conflict and friction. This could explain why it behaves differently from the diary Horcrux and also why it doesn't have the level of protections on it that the Gaunt ring did.
In the beginning of book 7 Hermione says that it is emotional closeness that puts you at risk from a Horcrux, not physical proximity, but we then see that wearing the locket has an effect on the Golden Trio members and that taking it off alleviates the effect. Furthermore, the locket does not seem to be as sentient or communicative as the diary.
Additionally, the locket is protected in a way that makes it challenging but possible to retrieve and even the potion that protects it, while unpleasant, is not poisoned (given that Kreacher survived drinking it with no ill effect). It's a perfect setup for a witness to make a "lucky" escape and for a determined outsider to then be able to retrieve the object based on the witness's information.
In contrast, the Gaunt ring was clearly something Tom did not want anyone touching or interacting with in any way. In addition to other protections, it had a curse on it that was meant to instantly kill anyone who touched it - a curse so powerful that even Dumbledore was not able to detect it before he put the ring on. The combined efforts of Dumbledore and Snape, two of the most accomplished and talented wizards ever, were not enough to counter this curse and the most they were able to do was just slow down its effects a little. (Also the fact that Dumbledore put the ring on so hastily suggests that there was a secondary curse that urged anyone who penetrated the other defenses to be tempted to put on the ring, which again used magic so sophisticated that he was not able to detect and disarm it). The ring's protections were so powerful and so deadly that they resulted in Dumbledore's death and would have probably killed any lesser wizard immediately.
Compared to that, the locket's defenses seem suspiciously easy to penetrate. And there is notably no fatal effects from handling the locket. In this context it is also possible that the fact that Kreacher could apparate out of the cave was not an oversight. Instead it could have been a plan to make the locket look like a Horcrux and make it seem as though Kreacher only escaped by chance. Tom may have presumed that Kreacher would tell Regulus and if Regulus was disloyal he would go after it because surely an object like that would be of great interest to the Order. Even the location he chose is a place that would peak the Order's interest since he might know that Dumbledore heard the story of that cave from Mrs. Cole.
If Regulus brought it to the Order to study it could then start sowing discord among them (just as it did with the Golden Trio) and interrupting their unity and operations. Furthermore, Regulus would be exposed as a spy.
Of course it goes a bit wrong because Regulus dies by suicide. However, Tom may have still assumed he was a spy, or at the very least disloyal and not bothered to retrieve the locket since Regulus's faithlessness plus Tom's general resentment against powerful pureblood families who once disdained him may have turned him against the Black family and he may have felt leaving it with them was a good punishment. (And then of course he lost his body shortly thereafter).
It could also explain why even after he realizes Harry is hunting Horcruxes he makes no attempt to retrieve the locket even though he doesn't know whether it's destroyed. (Of course, he may have simply assumed that Harry was using Fiendfyre to destroy any Horcruxes he found but this is a viable alternative). This could also mean that he still had other Horcruxes out there that were not identified.
(I am also entertained by the fact that the locket behaving kind of like the One Ring even works in-universe since given when LOTR was written Tom could've read it and been inspired lol.)
Anyway, I think it's a really fun alternate concept to think about.
