Why could <PERSON> open the door to the The Department of Mysteries when <PERSON> invaded his mind?
In the Order of the Phoenix, <PERSON> was always dreaming about a mysterious door. He had seen the door before but never had the chance to go inside. However, when <PERSON> used Legilimency on <PERSON> in one of their lessons, <PERSON> 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 –?
‘<PERSON>!’
<PERSON> 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 <PERSON>, who was standing over him, looking furious.
‘I ... 573
dunno what happened,’ said <PERSON> 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 <PERSON> was looking through <PERSON>'s eyes because of the Horcrux connection between them? <PERSON> 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 <PERSON> was planting fake memories in <PERSON>'s mind? Maybe neither <PERSON> nor <PERSON> had been inside the circular room in the DoM and he might just imagine the whole scene up according to what <PERSON> had described to him.
Which theory is more plausible? Or is there a third explanation? 573