Why did Voldemort need Hagrid's help to get past Fluffy?

Question

In the Sorcerer's Stone, Voldemort, acting through Quirrell, gets Hagrid drunk and "loses" a dragon's egg to him in order to find out how to get past Fluffy. That seems like incredible overkill. Surely Voldemort can teach Quirrell a sleeping spell, or just have Quirrell kill the dog outright. Not to mention the fact that dragons' eggs are incredibly rare and hard to get a hold of.

Answer 1

Fluffy being clearly alive and alert reduces suspicion that the Stone was in danger.

Fluffy was the first defense, and therefore also the one everyone would notice being incapacitated. Even Harry kept checking on Fluffy to know the status of the Stone.

“Every time they passed the third-floor corridor, Harry, Ron and Hermione would press their ears to the door to check that Fluffy was still growling inside. Snape was sweeping about in his usual bad temper, which surely meant that the Stone was still safe.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 14, (Norbert the Norwegian Ridgeback)}}

They were also listening for him growling, which meant a spell that didn't kill him but resulted in him being quiet would still cause suspicion.

“Yet the days crept by and there could be no doubt that Fluffy was still alive and well behind the locked door.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 16, (Through the Trapdoor)}}

If he suddenly got quiet, it would alert Harry and presumably also the staff that the Stone was in more serious danger. Leaving Fluffy meant not raising suspicion. It would be especially useful if it took multiple attempts to get to the Stone, and we do know of at least one other attempt.

“You’re too nosy to live, Potter. Scurrying around the school at Hallowe’en like that, for all I knew you’d seen me coming to look at what was guarding the Stone.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 17, (The Man with Two Faces)}}

If Fluffy had been killed or seemed incapacitated after that, it's likely the Hogwarts staff would have increased their defenses or even moved the Stone entirely. As it was, Fluffy was alert enough to maul Snape's leg, and Snape was probably the only staff member that suspected what was going on.

Even presuming his success, it would be highly beneficial to Lord Voldemort if he was able to get the Stone without anyone noticing, and have the Hogwarts staff diligently guarding nothing, unaware he would be strengthened by the Elixir of Life.

As it is, the only one of the defenses that was already handled before Harry got to it was the troll, which was further down the line of defense, and therefore would be less noticeable.

Many magical creatures are resistant to magic, and Fluffy might be as well.

There isn't any more information about three-headed dogs specifically, either in the seven books or in "Fantastic Beasts and Where to Find Them" (either version of the book), but from what we know about other magical creatures, it's quite possible that they might be resistant to magic. Dragons, Graphorns, and Manticores are all resistant to magic. Casting a sleeping spell on them would likely be difficult or even close to impossible.

Many magical creatures are very difficult to fight even if they're not really resistant to magic.

It's never mentioned in "Fantastic Beasts and Where to Find Them" if Acromantulas, Chimaeras, Erumpents, Griffins, Nundus, Quintapeds, trolls, and yetis are actually resistant to magic. However, they are all highly dangerous, and some require either very powerful magic or multiple wizards working together to subdue them. It's unknown exactly how hard it would be to fight Fluffy using magic, but Hagrid does imply that Fluffy is harder to take care of than a dragon would be.

“So I told him, after Fluffy, a dragon would be easy …”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 16, (Through the Trapdoor)}}

Hagrid often underestimates the danger of magical creatures, but this gives us at least a good comparative sense of how dangerous Fluffy might be.

Lord Voldemort was still weak, not at his full strength, and Quirrell was weakening as well.

Lord Voldemort was in a fairly weak state when he was possessing Quirrell. Although he had a servant to possess, and was strengthened somewhat by unicorn blood, he was still a wraith and nowhere near his most powerful. Quirrell even doubts that he's strong enough to be revealed to face Harry.

“Let me speak to him … face to face …’

‘Master, you are not strong enough!’

‘I have strength enough … for this …”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 17, (The Man with Two Faces)}}

Lord Voldemort even says himself to Harry that he's been greatly weakened, and needs the Stone to regain a body, and with it his own power.

“See what I have become?’ the face said. ‘Mere shadow and vapour … I have form only when I can share another’s body … but there have always been those willing to let me into their hearts and minds … Unicorn blood has strengthened me, these past weeks … you saw faithful Quirrell drinking it for me in the Forest … and once I have the Elixir of Life, I will be able to create a body of my own …”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 17, (The Man with Two Faces)}}

Quirrell was also likely getting progressively weaker due to being possessed, as the animals that the Dark Lord possessed did.

“I sometimes inhabited animals – snakes, of course, being my preference – but I was little better off inside them than as pure spirit, for their bodies were ill-adapted to perform magic … and my possession of them shortened their lives; none of them lasted long …”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

We see evidence that Quirrell is indeed growing weaker in this way.

“Quirrell, however, must have been braver than they’d thought. In the weeks that followed he did seem to be getting paler and thinner, but it didn’t look as though he’d cracked yet.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 14, (Norbert the Norwegian Ridgeback)}}

It was comparatively easier to get information from Hagrid than try fighting Fluffy without it.

Considering all factors, it would be much easier and more logical for Lord Voldemort to get Hagrid to accidentally mention how to get past Fluffy than for him and Quirrell to fight Fluffy without this knowledge. Getting information from Hagrid is very easy, and playing a harp is much less effort than spells which may have little effect anyway.

Answer 2

There is no direct reference in the books, so all in-universe explanations are my assumptions and deductions.

Out-of-universe reason: protection of the Stone was meant to be a quest for Harry and his friends, as well as for Voldemort. Also, there was a need to expose a particular trait in Hagrid's character - love for dangerous beasts.

In-universe reasons: - Knowing Hargid, I would assume that Fluffy would be tough to disable or kill with magical methods, like Hagrid himself. Also, Quirrell was not as good at using magic as Voldemort, and Voldemort was weak, relying on Unicorn blood to keep his strength. He needed his strength for a possible encounter with someone possibly more powerful than Fluffy. If Voldemort/Quirrell failed to kill or disable Fluffy in one strike, he would put up a fight. The whole castle would be alert, which was not something Voldemort wanted.

Answer 3

Consider these facts:

• Voldemort was weak - Having not regained his body yet, Voldemort was simply a disembodied face on the back of Quirrel's head. He was not capable of using his own powerful magic to neutralise the dog somehow.
• Voldemort's host body was Quirrel - We don't know for sure whether Quirrel, who was providing Voldemort's only means of getting past Fluffy by being his host body, would be capable of using magic to stop Fluffy. The dog was generally portrayed as being very frightening and powerful, and Quirrel was generally portrayed as being a weak, stuttering and timid man. Therefore I think evidence points to Quirrel's magic not being strong enough to stop Fluffy, even if Voldemort instructed him on potential powerful spells.
• Voldemort needed to find out how to stop Fluffy in a way that did not draw attention to himself - Voldemort couldn't do anything that would arouse Hagrid's suspicion about Quirrel, in case of a follow up investigation at Hogwarts. Voldemort was planning on getting the Philosopher's stone, and the best way of doing that would seem to be quietly sneaking into the third floor corridoor without causing anyone any reason to suspect until the moment he enacted his plan. Doing so by having Quirrel pretend, as a fellow Hogwarts staff member, to socialise with Hagrid, get drunk together and lead Hagrid to spill the secret, would not be a bad way of doing so, as Hagrid would probably be slightly embarassed by the encounter but otherwise not think much of it.

With these considerations I think the method Voldemort used to get the key to stopping Fluffy made sense, and was not necessarily overkill.

Answer 4

I think that the Dog has also some sort of Protection in his blood. Like a Giant or a Dragon. This means you can't just kill Fluffy. In Harry Potter and the Goblet of Fire, they couldn't set the dragons to sleep with only one spell. They did it with 3-5 people.

In the Harry Potter Wiki there are some paragraphes about spell resistance. In there is mentioned that Dragons have some resistance because of the Skin. And also in the book Harry Potter and the Prisoner of Azkaban, where some Wizards try to stop Hagrid.

Ron: "Hagrid did well, didn't he? How come all the spells bounced off him?"
Hermione: "It'll be his giant blood. It's very hard to Stun a giant, they're like trolls, really tough... but poor Professor McGonagall... four Stunners straight in the chest and she's not exactly young, is she?"
_{Ron and Hermione discuss Hagrid's surviving multiple Stunning Spells}