In the Order of the Phoenix, Harry was always dreaming about a mysterious door. He had seen the door before but never had the chance to go inside. However, when Snape used Legilimency on Harry in one of their lessons, Harry was forced to see the door open, which he'd never dreamed about:

He was hurtling along the corridor towards the Department of Mysteries, past the blank stone walls, past the torches – the plain black door was growing ever larger; he was moving so fast he was going to collide with it, he was feet from it and again he could see that chink of faint blue light –

The door had flown open! He was through it at last, inside a black-walled, black-floored circular room lit with blue-flamed candles, and there were more doors all around him – he needed to go on – but which door ought he to take –?


Harry opened his eyes. He was flat on his back again with no memory of having got there; he was also panting as though he really had run the length of the Department of Mysteries corridor, really had sprinted through the black door and found the circular room.

‘Explain yourself!’ said Snape, who was standing over him, looking furious.

‘I ... dunno what happened,’ said Harry truthfully, standing up. There was a lump on the back of his head from where he had hit the ground and he felt feverish. ‘I’ve never seen that before. I mean, I told you, I’ve dreamed about the door ... but it’s never opened before ...’ (Harry Potter and the Order of the Phoenix, UK edition, pp.522-523)

Does this suggest Harry was looking through Nagini's eyes because of the Horcrux connection between them? Nagini might be lurking around the Department of Mysteries and manage to get inside, but I haven't found any canon to support that yet.

So is it possible that at that point Voldemort was planting fake memories in Harry's mind? Maybe neither Voldemort nor Nagini had been inside the circular room in the DoM and he might just imagine the whole scene up according to what Rookwood had described to him.

Which theory is more plausible? Or is there a third explanation?


Because Voldemort knew what was inside

First, we must recognize that (aside from the final vision, that of Voldemort torturing Sirius) the dreams that Harry had were not planted by Voldemort. They were simply a natural consequence of the bond between Harry and Voldemort.

“Professor Snape discovered,” Dumbledore resumed, “that you had been dreaming about the door to the Department of Mysteries for months. Voldemort, of course, had been obsessed with the possibility of hearing the prophecy ever since he regained his body, and as he dwelled on the door, so did you, though you did not know what it meant.

Harry Potter and the Order of the Phoenix

Since Voldemort was constantly thinking about the door, Harry saw it in his dreams.

Further, Snape purposely tried to hurt Harry during his Occlumency lessons, due to his dislike for Harry. As Harry noted, this actually increased his vulnerability to Voldemort's influence (conscious or unconscious):

“Snape made it worse, my scar always hurt worse after lessons with him —” Harry remembered Ron’s thoughts on the subject and plunged on. “How do you know he wasn’t trying to soften me up for Voldemort, make it easier for him to get inside my —”

“I trust Severus Snape,” said Dumbledore simply. “But I forgot — another old man’s mistake — that some wounds run too deep for the healing. I thought Professor Snape could overcome his feelings about your father — I was wrong.”

Harry Potter and the Order of the Phoenix

In other words, during his Occlumency lessons Harry was more receptive to Voldemort's thoughts.

The scene in question took place right after Harry had used a Shield Charm to reverse Snape's Legilimency on him. As such, Harry believed that Snape was going to punish him for his actions (presumably by invading his mind painfully).

Harry felt a thrill of dread: He was about to pay for what had just happened, he was sure of it. They moved back into position with the desk between them, Harry feeling he was going to find it much harder to empty his mind this time....

Harry Potter and the Order of the Phoenix

Snape was loyal to Dumbledore, but he hated Harry. He enjoyed making Harry look inadequate, to the point where he would tear down Harry's mental defenses out of sadism. He had probably just done this before Harry had his vision of the Department of Mysteries. This allowed Voldemort's thoughts to come through more easily. As such, this new vision was not a memory, but a picture of what Voldemort was thinking about at that moment.

How did Voldemort know what was in the Department of Mysteries?

  • He might have been there for some reason when he was younger.
  • More likely, he had acquired the information directly from the mind of Rookwood, who had been an unspeakable and had presumably been at least in the antechamber.

Harry has had be asleep whenever Voldemort tried to plant the vision of the Department of Mysteries inside Harry's mind, Harry had never even seen nor heard of that place before. So the fact that he could see that corridor and what was inside it has got nothing to do with Harry. He could always open that door and see what was inside, as he was always seeing what Voldemort wanted to plant in his mind, but whenever Voldemort was about to show him what was inside the door in the previous occasions, he had always woke up, breaking the connection between his mind and Voldemort's. During the legilimency attacks however, his mind seemed to be more vulnerable to the visions from Voldemort and the connection was there long enough for Voldemort to be able to just show what was inside the door. Harry would probably have seem more, had it not been for Professor Snape breaking the connection.

During the battle at the Ministry it is hinted that Voldemort never really was there when Harry saw him torturing Sirius. This seems to indicate that he doesn't really need to be there in order to plant visions of that place in Harry's mind. The only time he was really there was during the attack on Arthur Weasly. He could probably plant any images he wished whenever Harry wasn't completely conscious.


While it is true (as a previous answer has pointed out) that sometimes Harry was startled awake before he would have had a chance to open the door, that appears to have just been coincidental. He had the dreams almost every night and until that session with Snape he had never been able to open the door, even when he wasn't startled awake:

What was more, he was now dreaming about walking down the corridor toward the entrance to the Department of Mysteries almost every night, dreams that always culminated in him standing longingly in front of the plain black door.

I believe there is a simple explanation as to why he suddenly was able to open the door. First, though, we must note that he was able to open the door in subsequent dreams as well, for instance:

He had fallen right into the corridor leading to the Department of Mysteries. He was speeding toward the plain black door... Let it open... Let it open... It did. He was inside the circular room lined with doors... He crossed it, placed his hand upon an identical door, and it swung inward...

So it appears that there was a turning point in Harry's dreams. Up until the session with Snape he had been unable to open the door; from that session and on he was able to open the door.

So what changed?

The answer is, I believe, that Voldemort changed. The key passage here comes from the end of the book, when Lucius Malfoy explains about the prophecy (my emphasis):

“Dumbledore never told you?” Malfoy repeated. “Well, this explains why you didn’t come earlier, Potter, the Dark Lord wondered why —”

“— when I say go —”

“— you didn’t come running when he showed you the place where it was hidden in your dreams. He thought natural curiosity would make you want to hear the exact wording...”

I think Lucius was saying that Voldemort deliberately showed Harry the place in his dreams in order to get him to come retrieve it. However, this deliberate showing was a late development.

For the first half of the year Voldemort did not realize that he had a mind connection with Harry. As such, everything that Harry saw was unintentional from Voldemort's perspective; Harry was simply seeing what Voldemort's mind was preoccupied with.

This is explicitly stated by Snape during the first Occlumency lesson:

“It appears that the Dark Lord has been unaware of the connection between you and himself until very recently. Up till now it seems that you have been experiencing his emotions and sharing his thoughts without his being any the wiser. However, the vision you had shortly before Christmas —”

However, when Harry saw the attack on Mr. Weasley, he penetrated far enough into Voldemort's mind that Voldemort found out about the connection. This too is stated by Snape during the first Occlumency lesson:

“It is enough that we know,” said Snape repressively. “The important point is that the Dark Lord is now aware that you are gaining access to his thoughts and feelings. He has also deduced that the process is likely to work in reverse; that is to say, he has realized that he might be able to access your thoughts and feelings in return —”

It was only from this point on that Voldemort was able to use the connection to manipulate Harry, because it was only from this point on that he knew about the connection.

Now Voldemort obviously knew what the Department of Mysteries looked like, or else Harry would never have been able to get past the door. However, during the time before Voldemort realized there was a connection, the door never opened because that is what Voldemort's mind was occupied with. I.e. he was trying to find a way to get into the Department of Mysteries. What lay beyond the door was irrelevant to his thoughts at the time. All that mattered was getting through the door.

When he realized there was a connection he was no closer to being able to get through the door. However, at that point he realized he could use Harry by playing to his curiosity. Merely seeing a corridor with a door at the end would hardly be enough too get Harry to do anything foolish, so Voldemort upped the ante. He began showing Harry what lay beyond the door, giving him very specific looks into the Department of Mysteries so that Harry would be able to figure out how to retrieve the prophecy, as in this dream:

Last night he had once again made the journey along the Department of Mysteries corridor. He had passed through the circular room, then the room full of clicking and dancing light, until he found himself again inside that cavernous room full of shelves on which were ranged dusty glass spheres...

He had hurried straight toward row number ninety-seven, turned left, and ran along it... It had probably been then that he had spoken aloud... Just a bit farther... for he could feel his conscious self struggling to wake... and before he had reached the end of the row, he had found himself lying in bed again, gazing up at the canopy of his four-poster.

It just happened to be that Voldemort's plan didn't work so well, as Lucius acknowledged, because Harry didn't know about the prophecy to begin with.

So in short, there was nothing special about the particular session with Snape that allowed Harry to open the door. It was simply that up to that point the door had never opened because Voldemort, not realizing that Harry was watching, didn't care what was beyond the door. It was only once he realized that Harry was watching that he began to deliberately show Harry what was beyond the door.


I theorise at this point, Voldemort had already started reverse planting the images into Harrys mind, and it wasn't Harry's own doing that allowed him to see the door open, but it was already planted there by Voldemort. Voldemort himself may have seen the door (or sent Nagini) at a different point beforehand to get the images.

Using Legilimency, Snape could see and drill into Harry's mind and flashes of things even Harry himself didn't remember he had from his past. So when Snape used "Legilimens" on Harry to penetrate his mind, and saw the door open, Snape must have known that Voldemort was able to plant images deep into Harrys mind (yet Harry was unaware of them still), which is exactly what the Order feared, and why Snape was so angry.

tldr; Voldemort planted door opening in Harrys mind at one point before the lesson, Snape found out via Legilimency, confirming Order suspicions that Voldemort could plant into Harrys mind.

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