Does Voldemort believe that he is the only one who knows about the Room of Requirement

Question

In HP7, Harry, Hermione, and Ron manage to steal the Hufflepuff cup from Gringotts. On hearing this, Voldemort's in a rage and once he cools down, he starts thinking about visiting and making sure the other Horcruxes are safe. This is what the book tells us then:

As for the school: He alone knew where in Hogwarts he had stowed the Horcrux, because he alone had plumbed the deepest secrets of that place...

Harry is in Voldemort's head at this time. So he knows the Horcrux (which he shortly afterwards finds out to be Ravenclaw's diadem) is at Hogwarts and when he's trying to think where it might be hidden, this is the relevant passage:

And as he reached the end of the passage, the memory of a third stone effigy came back to him: that of an ugly old warlock, onto whose head Harry himself had placed a wig and a battered old hat. The shock shot through Harry with the heat of firewhisky, and he nearly stumbled. He knew, at last, where the Horcrux sat waiting for him. . . . Tom Riddle, who confided in no one and operated alone, might have been arrogant enough to assume that he, and only he, had penetrated the deepest mysteries of Hogwarts Castle.

And this is what he tells Hermione and Ron:

“I know what the diadem looks like, and I know where it is,” said Harry, talking fast. “He hid it exactly where I had my old Potions book, where everyone’s been hiding stuff for centuries. He thought he was the only one to find it. Come on.”  As the walls trembled again, he led the other two back through the concealed entrance and down the staircase into the Room of Requirement.

But in HP6, Draco was assigned the task of repairing half of the vanishing cabinet in the Room of Requirement so that Death Eaters could enter Hogwarts from Borgin and Burkes. Now, how could Voldemort still think that no one else knew about the Room of Requirement in HP7 and assume that "he alone plumbed the deepest secrets of the place"?

I know there are other similar questions here, specifically this. But those can all, at a stretch, be chalked up to a.) Voldemort being arrogant, or b.) Voldemort actually being the first one ever to use the room. My question is about inconsistency in his own behavior: on the one hand, assigning the task to Draco and on the other thinking he alone knew about the room.

Answer 1

Voldemort definitely knew that the Room of Requirement had other visitors.

There have been two things that would tip him off:

• The DA meetings. Although he spends most of Order of the Phoenix avoiding Harry, he’d be very interested in what he was up to – particularly if he starts forming an anti-Voldemort club. Probably not a serious threat, but something to keep an eye on.

  Most of the Inquisitorial Squad (all?) are Slytherins, and that includes the children of several Death Eaters. They’re an easy source of intel on Harry, and at some point he’d recognise their headquarters as the Room of Requirement.

• Draco’s mission. Voldemort doesn’t seriously expect Draco to succeed; he expects him to die in the attempt. Hogwarts has never been successfully breached – including by Voldemort himself.

  So when Draco looks like he might succeed, Voldemort would be very interested. He’ll want details. Draco knows exactly what the room is called, and presumably, what it does – he names it to Dumbledore – so Voldemort will know that the secret is out.

So how do we square this with Voldemort’s thinking? On two counts:

• Even if the Room of Requirement is known, nobody knows what it contains:

  He alone knew where in Hogwarts he had stowed the Horcrux

  Despite having a parade of visitors, there’s no suggestion that anybody thought it had a horcrux. Almost nobody even knows that the Room, or the Horcrux, exist. This isn’t an unreasonable assumption.

  And even if you know the Room of Requirement exists, you may not know about the Room of Hidden Things – for example, the DA members only ever saw it as a classroom.

• He probably does have an unusual knowledge of the castle.

  because he alone had plumbed the deepest secrets of that place...

  The Room of Requirement is hardly the most secret place in the castle – Slytherin’s Chamber tops that, and even that had a steady stream of visitors. He probably does know more than almost anybody else, and chose what he thought was the best hiding place. Most people wouldn’t know where to start looking, Room knowledge or otherwise.

Answer 2

Did Voldemort know the Vanishing Cabinet was in the same room as the diadem?

As TGnat’s answer and quotes show, the assumption in the question that Voldemort tasked Draco with using the Vanishing Cabinet is almost certainly incorrect. It was Draco’s own idea, and his own execution.

Voldemort himself was not—for whatever reason—involved in the events of that night when the Death Eaters were sneaked into the castle. He would, in other words, only have the reports of others (whether Draco himself or other Death Eaters) to rely upon.

Since Draco knew perfectly well what failure meant, it seems likely that he did not confide the details of his plan to anyone (we know for certain that he withheld it from Snape) until he had successfully repaired the Vanishing Cabinet and transported something to Borgin and Burkes and back again.

As it happens, we know with almost certainty that this happened almost immediately before Harry and Dumbledore left to go destroy the locket in the cave. In chapter 25, “The Seer Overheard” in Half-Blood Prince, we are first told that it is evening (my emphasis):

On one such evening, when Ginny had retired to the library and Harry was sitting beside the window in the common room, […] Hermione dropped into the seat between him and Ron with an unpleasantly purposeful look on her face.

During their conversation, Jimmy Peakes gives Harry a note from Dumbledore to come see him immediately. On his way there, Harry runs into Trelawney, who has just been forcibly thrown out from the Room of Hidden Things by someone male who was whooping with happiness. There’s a bit about Snape being the one who overheard half the original Prophecy, and then Harry goes to Dumbledore’s office, whence they set off for the cave at once.

This whooping, male Trelawney-ejector almost has to be Draco who has just managed to retrieve something from Borgin and Burkes—there would be no reason to include it in the story if it were anything else. Even Harry assumes this to be so in his conversation with Dumbledore before leaving for the cave. Assuming I am right that Draco told no one of his plan until he knew it would work, this means that Voldemort wasted no time in putting the plan into effect—it had been carried out mere hours after Draco sent word of it to him. The description and details of the plan were thus likely rushed through.

There is no evidence to suggest that Voldemort was ever told exactly where inside Hogwarts the newly-repaired Vanishing Cabinet was located. The plan was extremely simple: get Death Eaters to Borgin’s and Burke’s, have them go into the Vanishing Cabinet, and they will appear at Hogwarts, whence they will be free to wreak havoc and secure the way for Draco to kill Dumbledore.

In describing that plan to Voldemort on a tight schedule, there are many details that are more important than the precise location of the Vanishing Cabinet: where Dumbledore was, how to lure him back, who should play which role in wreaking havoc, where to lure Dumbledore to, etc. Even if the location of the Cabinet was mentioned in all this, it is not unlikely that it would be in more generic and practical terms, mentioning the location of the room (“once the Death Eaters are through, they’ll come out in a room on the seventh floor in the eastern tower”), rather than describing in detail what the actual room itself is, which is really quite irrelevant to the plan itself.

As it is we have no evidence to either prove or disprove that Voldemort knew the Vanishing Cabinet was in the Room of Requirements, but there’s at least a good case to be made that he didn’t.

 

Does it matter?

Quite apart from that, though, he was right: despite all the students who had been using the Room of Hidden Things over the years since he hid the diadem there, no one did know that that’s where it was. Even Harry, who had actually handled the diadem with his own two hands, had not recognised it and did not know where it was, despite the fact that he was actively searching for it.

There’s a good chance that, even if he realised that the Vanishing Cabinet was in the same room as the diadem, he would not be overly shaken by the fact. His method of security through obscurity was as good as it had ever been, and the fact that others had also found some use for the (enormous) room did not really change that. He alone did know where he had put the diadem, at the time.

And very importantly: he is trying to calm himself down by convincing himself of the unlikelihood of anyone knowing about any of the Horcruxes. He is enumerating and exaggerating reasons why no one could possibly know about them. He mentions the ring, the locket, and the diadem, and the other two are also exaggerated:

No one had ever known him to be related to the Gaunts, he had hidden the connection

One had only to do what Dumbledore did and look into his orphanage past to find out where the name Tom Marvolo Riddle came from; not too difficult to discover.

And how could the boy, or anybody else, know about the cave or penetrate its protection? The idea of the locket being stolen was absurd …

Both Regulus (with Kreacher’s help) and Dumbledore (with Harry’s) managed it, nonetheless. Again, his orphanage past held the clue.

Speaking logically, even Voldemort would probably have to admit that all three statements are not absolute. It would be theoretically possible to discover his connection to the Gaunts; there was at least one or two ways to come to know about the cave; and he couldn’t know for sure that others did not have as intimate knowledge of Hogwarts’ secrets as he did.

But Voldemort is not thinking logically at the moment: he is convincing himself, calming himself down, and using his exaggerated and arrogant sense of superiority as a tool to do so.

Answer 3

It is unlikely that Voldemort knew the exact details of Draco's plan.

Draco explains to Dumbledore in HBP that he got the idea from Montague who had gone missing the previous year:

I had to mend that broken Vanishing Cabinet that no one’s used for years. The one Montague got lost in last year.

...and they make a kind of passage between them. Montague told me that when he was stuck in the Hogwarts one, he was trapped in limbo but sometimes he could hear what was going on at school, and sometimes what was going on in the shop, as if the cabinet was traveling between them

And then he takes full credit for the idea:

I was the one who realized what it meant - even Borgin didn't know - I was the one who realized there could be a way into Hogwarts through the cabinets if I fixed the broken one.

So he was not specifically tasked with fixing the cabinet, it was a Draco original...