Why did Voldemort need a house-elf?

Question

(I’m currently travelling with no access to the books, so bear with my paraphrasing from memory.)

During Deathly Hallows, we see through Kreacher’s retelling how –

• Voldemort told his Death Eaters he needed a house-elf to assist him in a task
• Regulus volunteered Kreacher for the task and told him to come back after the task was finished
• Voldemort took Kreacher to the cave by the sea, made him drink that rather unsavoury potion that Regulus and Dumbledore later both drank, too
• Voldemort placed the locket in the stone basin when the potion was all drunk
• Voldemort left Kreacher to be killed by the Inferi (not realising that his anti-Apparition spells did not affect elf magic)

So far so good. Kreacher escaped and told Regulus the whole ordeal; Regulus had a change of heart, managed to get to the cave with Kreacher, got the real locket out and exchanged it for a fake one, then died at the hands of the Inferi (perhaps unnecessarily), Kreacher once again coming back, this time with the real locket.

 

Now, the cave itself seems to be a natural cave: it was there when Riddle went there as a child). But most of the ‘extra features’ in it were presumably created by Voldemort himself later on, quite likely with the specific purpose of hiding a Horcrux there. At least the stone basin and the potion within it doesn’t seem to have had any other purpose than the locket.

We know that Voldemort is able to get to the basin and examine its contents quite easily (as seen by Harry during Deathly Hallows), which makes sense—they’re all his own spells, after all, so he would have built in ‘back doors’ for himself. But that must mean that he was able to somehow drain the potion from the basin without drinking it, or summoning the locket from within the potion—something that no one but he could do.

 

So the real question:

If Voldemort made the basin and potion himself with the purpose of hiding a Horcrux in it, why did he need a house-elf to drink the potion in order to hide the locket there? Why not just put the locket in the basin before filling it with the potion? Or indeed just use one of his ‘back doors’ to put the locket in the potion the same way he later checked whether it was still there.

That way, there would have been no witnesses at all, and no way for anyone to know where the locket was. Why bring an unnecessary witness? Just to do a test run and see if his potion worked? Doesn’t seem like how Voldy’s mind works—he’s the greatest wizard ever, of course his magic will work. In fact, given that he could come back and test it himself with no problems, why have the potion work like that at all, enabling just about anyone with enough will-power (and a helper) to retrieve the locket?

Answer 1

To take it in order:

• Why not just put the locket in the basin before filling it with the potion?

It seems likely that the presence of such strong magic as a Horcrux would have interfered with the spells used to create the basin.

But even if that isn't true, Voldemort would undoubtedly (as Harry speculated) have wanted a test run of the cave's defenses and the way through them before trusting the locket to them. After all, what if it hadn't worked as planned, and Kreacher had proved unable to empty the basin? That would likely mean that the defenses were flawed, and if the locket was already behind them, Voldemort would have to expend considerable effort (and perhaps risk) to tear them down again to get it out.

Conversely, of course, if they were never tested then Voldemort could not be certain they were as strong as he had intended. And as far as Voldemort was concerned, Kreacher was entirely expendable; his death was a trivial price to pay to ensure the safety of part of Voldemort's soul.

• Or indeed just use one of his ‘back doors’ to put the locket in the potion the same way he later checked whether it was still there.

There were no back-doors, or rather, the use of a house-elf was the back door; the plan, in my opinion, was that if he ever needed to remove the locket he'd "borrow" another house-elf. That was, no doubt, a protection against Dumbledore, who would surely have been able to discover any other weaknesses the spell might have been given but would have been too ethical to think of sacrificing another living being in such a way.

Of course, there was a flaw in the plan, but Voldemort couldn't have imagined that Dumbledore would be willing to sacrifice himself.

• We know that Voldemort is able to get to the basin and examine its contents quite easily

Examine the contents, but not remove the contents. From Chapter 29 of Deathly Hallows:

As she said it, a wrath that was like physical pain blazed through Harry, setting his scar on fire, and for a second he looked down upon a basin whose potion had turned clear, [...]

(emphasis mine).

It is clear (no pun intended) from this that the potion was still there, and hence still protecting any contents the basin might have had - we can therefore conclude, as suggested in the comments, that this isn't a back door so much as a form of tamper-evident seal.

Additional: this quote also shows that the potion is indeed, as I had assumed, an implicit part of the basin's enchantment rather than a separate spell, since at some point between when it was emptied in Half-Blood Prince and this scene in Deathly Hallows it has refilled itself without Voldemort's intervention. (Kreacher's description of his encounter with the basin suggested otherwise, but under the circumstances he may not have completely understood what was happening.) [And as pointed out in the comments, it must also have refilled itself after Regulus emptied it, or else Harry and Dumbledore would have found it empty.]

That may mean that the basin and potion had to be created together, and that the only way to store something in the basin was to have someone drink the potion first - but of course in the absence of a qualified wizard, this is still just speculation. :-)

As for why the potion wasn't lethal, Dumbledore explains this in Half-Blood Prince, chapter 26:

'I'm sorry, Harry; I should have said, he would not want immediately to kill the person who reached this island,' Dumbledore corrected himself. 'He would want to keep them alive long enough to find out how they managed to penetrate so far through his defences and, most importantly of all, why they were so intent upon emptying the basin. Do not forget that Lord Voldemort believes that he alone knows about his Horcruxes.'

(It isn't clear to me how this fits in with the presence of the Inferi, but perhaps they were attempting to capture rather than kill, all appearances to the contrary.)

Answer 2

Harry seems to think (and we're given no reason to doubt him) that it was indeed a test run of the cave's defenses.

Of course there had been a boat; Harry knew the boat, ghostly green and tiny, bewitched so as to carry one wizard and one victim toward the island in the center. This, then, was how Voldemort had tested the defenses surrounding the Horcrux, by borrowing a disposable creature, a house-elf…

In particular, I think Voldemort was confident that the locket was protected, but he needed to test whether he'd be able to retrieve the locket later. After proving that his hidden boat would carry him and an elf and that the potion wouldn't kill the elf before he drank it all, he felt comfortable leaving the locket.

Answer 3

Your key point here is that

they’re all his own spells, after all, so he would have built in ‘back doors’ for himself

First of all, Voldemort might just be cruel enough to make Kreacher drink the potion even if he could have simply Summoned the locket, just to torture him, but I don't think this was his reason. I can think of a few other reasons:

• He could have just wanted to see if the potion worked. He is the greatest wizard ever, maybe, but it still benefits him to be careful, to make sure that the potion is working well enough to guard the locket and make the people who drank it crazy/in pain.

• He could have needed something to be bait for the Inferi--maybe they get agitated every time someone comes to the cave, and he needed Kreacher to use to be Inferiated so that Voldemort could leave safely.

• Maybe he didn't build 'back doors' at all. It's very likely that if he had, someone else--Dumbledore, for instance, who later had to drink the same potion--could have figured out how to get around drinking the potion, which would have basically gone against the whole point of him being there. Or he just never thought he'd need to use them, since he knew that if he needed to get past the potion, he would have plenty of followers and their house-elves to force to drink the potion.

Answer 4

He didn’t need to retrieve the locket in Deathly Hallows, he just needed to ensure it was still there. If it was there, it was safe and if it was gone it was too late. If he wanted to move it somewhere safer he could always go back and find a house elf but his goal when he went to the cave was to find out if it was still there, no house elf necessary.