(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?