Voldemort didn't expect Draco to succeed, so perhaps he had just assigned Draco to the task and let him do whatever he wanted
So most probably he didn't know about the cursed necklace nor the poison Draco sent to Professor Slughorn
However, at the end of the Half Blood Prince, Draco did manage to get top level Death Eaters such as Carrows and Bellatrix Lestrange, who seem to be very close to Voldemort, unseen into Hogwarts
Wouldn't Voldemort at least have been interested in knowing how Draco managed to do it?
If Voldemort knew about how Draco got Death Eathers into the school, it would have shown him immediately that the Room of Requirement can be and has been found by other people, and it wasn't just he who had found out about it
Would Voldemort really just ignore Draco's most significant attempt?
4 Answers
If Voldemort knew about how Draco got Death Eathers into the school, it would have shown him immediately that the Room of Requirement can be and has been found by other people, and it wasn't just he who had found out about it
First of all, to the best of my knowledge, there's no canon mention of Voldemort knowing about details of the plan, so the rest is purely speculation.
Having said that, Voldemort was a high level manager. Quite possibly, he would not have bothered with minor detail like "which room was the Vanishing Cabinet in", even if he - quite possibly - knew about the high level details of the plan like using the Cabinet.
Also, the Room of Requirement was different for everyone. A "store all your junk" storeroom that Voldemort likely saw may not have immediately associated with "room that had waterclosets" that Dumbledore found, with "Room to have the meetings in" that DA used. So Draco may very well have seen - and described - merely "A room where Vanishing Cabinet is stored" without realizing what that room was or mentioning any other things that would let someone else associate it with Room of Requirement.
Harry (and from him, DA) knew about the detailed properties of the room only because Dobby told him.
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Also, as Draco was at least somewhat proficient at occlumency, perhaps the Dark Lord knew less than he thought.– user11521May 30, 2014 at 20:49
I don't have my books to hand but I'm pretty sure we don't hear in exact words what Draco was tasked with. However it can be presumed that he was tasked to kill Dumbledore.
The books do not mention that Voldemort knew what Draco was doing as he also did not know that his mother Narcissa Malfoy had made the unbreakable vow with Snape, but did tacitly expect Snape to kill Albus if (as he expected) Malfoy was killed or incapacitated.
In his discussion with Dumbledore at the top of the Astronomy Tower, Malfoy states that he came up with the idea to use Vanishing Cabinets after Montague was shoved into the one in the fifth-floor corridor by Fred and George Weasley in OoTP.
Everyone thought it was a really good story, but I was the only one who realised what it meant - even Borgin didn't know - 1 was the one who realised there could be a way into Hogwarts through the Cabinets if I fixed the broken one.'
Draco Malfoy to Albus Dumbledore
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1I think it's mentioned in the last book, in Professor Snape's memories, that Draco was assigned to assasinate Dumbledore Feb 25, 2014 at 13:52
An additional point to the other good answers here:
Voldemort had learned during Harry's fifth year that Harry was capable of entering his mind and reporting to Dumbledore what he saw (e.g., Nagini's entrance into the Department of Mysteries). Despite employing Occlumency after the battle at the Ministry, it's possible that with Dumbledore's assistance and training (which he likely knew Harry was receiving, though not the content), Harry could have been able to break down the barriers between them and continue to spy on Voldemort. (Note that Voldemort appeared to be unaware of Harry's presence in his own mind until the night of Nagini's infiltration.)
Thus Voldemort had good reason to give simple orders and leave the details to his associates - learning the details would potentially give Harry insight and allow him to warn Dumbledore and prepare against the forthcoming invasion.
Voldemort himself tasked Draco Malfoy to kill Dumbledore. That much we know, however we can make educated guesses of the rest. It's quite likely Voldemort did not know any of the details of Draco's plans, and may have been out of the loop completely. This is mainly because Voldemort did not seriously task Draco to kill Dumbledore, assigning Draco on what he saw as an impossible task was his form of punishment for the failure of Lucius in the Department of Mysteries battle in "The Order of the Phoenix". Voldemort expected Draco to either get caught and thrown into Azkaban, or simply give up, which would justify Voldemort in killing him (the books do not clarify why Voldemort seemed to feel obliged to have a visible reason to execute Draco, but perhaps he simply found it a more satisfying punishment, to watch Draco's parents live in fear for their son for his own amusement).
Because Draco's mission was not of any actual importance, it's unlikely Voldemort bothered keeping track of what Draco was doing. He likely had other concerns occupying his time, and thus left communicating with Draco to a subordinate death eater. Draco's plan did not seem like it was going to work out until he made a breakthrough one night and fixed the vanishing cabinet in the Room of Requirement. When he fixed it, he likely contacted his death eater handler, who in turn assembled the local death eaters to his cause.
Since Voldemort did not make an appearance himself at all during the raid of Hogwarts, it's very likely he was otherwise occupied elsewhere, possibly abroad. Otherwise one would expect he might have taken advantage of Hogwarts' defenselessness (Dumbledore being the primary protection), and killed as many teachers and students as possible. However, he was not able to respond quickly enough, so the death eaters in the raid had to flee, as no doubt aurors rapidly would begin apparating as close as possible as soon as someone in Hogwarts was able to alert them of the invasion. By the time the death eaters regrouped, Professor Flitwick would probably have recast most of the castle's defense spells (which in all likelyhood were previously cast and maintained by Dumbledore, but fell with his death), and the castle would have been host to a small army of aurors.
