Horcrux Cave (Crystal Cave)
The Horcrux Cave, also known as the Crystal Cave, makes it's appearance in the sixth instalment in the Harry Potter series, "Harry Potter and the Half Blood Prince". Albus Dumbledore and Harry Potter travel to the Horcrux Cave searching for one of seven Horcruxes created by Lord Voldemort. Tom Riddle used the cave to torture two children and returned as Lord Voldemort to use the cave as a hiding place for one of his Horcruxes.
The Horcrux Cave was significant to the child Tom Riddle who later evolved into Lord Voldemort. The Horcrux Cave is located in England and resides by the sea. It is thought that as a child, Tom Riddle discovered the cave during a camping trip with other children from his orphanage. Tom Riddle took two children down to the cave, which we assume was done by magic because the cave entrance is described as inaccessible by boats or climbing down the face of the mountain. Tom proceeded to terrorise these children therefore rendering the cave as symbolic to himself later on in life. After splitting his soul and creating his Horcrux, Salazar Slytherin's locket, he hid it inside the cave and protected it with various magical enchantments.
"They bought the kids from the orphanage here?" asked Harry, who could not imagine a less cosy spot for a daytrip. "Not here, precisely", said Dumbledore. "There is a village of sorts about halfway along the cliffs behind us. I believe the orphans were taken there for a little sea air and a view of the waves. No, I think it was only ever Tom Riddle and his youthful victims who visited this spot. No Muggle could reach this rock unless they were uncommonly good mountaineers, and boats cannot approach the cliffs; the waters around them are too dangerous. I imagine that Riddle climbed down; magic would have served better than ropes. And he brought two small children with him, probably for the pleasure of terrorising them. I think the journey alone would have done it, don't you?" Harry looked up at the cliff again and felt goosebumps. - Harry Potter and the Half-Blood Prince, Chapter 26 pg. 519/20
 Inside the Horcrux Cave
Harry and Dumbledore proceed to enter the cave to retrieve what they believe to be one of the seven Horcruxes created by Lord Voldemort. However Voldemort cast a number of different enchantments in order to protect the Horcrux.
- First and foremost Lord Voldemort made it impossible for anyone(except himself) to have the ability to apparate inside the Horcrux Cave. In the book and the film adaptation Harry and Dumbledore apparate onto a rock outside the cave entrance. (see image up top) This enchantment does not prevent house elves from apparating within the cave, proven with the theft of the Horcrux by Regulus Black and his house-elf Kreacher. This loophole existed because Voldemort undermined the power of house elves and did not expect a house-elf to ever be anywhere near the Horcrux Cave.
- There is an enchantment placed upon the door to the cave which Voldemort intended to weaken trespassers with. The door requires a small blood sacrifice to open. Dumbledore explains how once again Voldemort made a misjudgement, this being that physical pain would suffice in weakening a trespasser.
"The idea, as I'm sure you will have gathered, is that your enemy must weaken him or herself to enter. Once again, Lord Voldemort fails to grasp that there are much more terrible things than physical injury."
- A lake separates the entrance of the cave from the island on which the horcrux is placed. Within the lake lie Inferi. Inferi are corpses which have been enchanted to carry out their master's bidding. The inferi within the cave's lake are there to pull in anyone who disturbs the water and drown them. Additionally, attempting a summoning charm to obtain the Horcrux causes some of the Inferi to leap out of the water. In the Half-Blood Prince (book) Harry attempts a summoning charm and an Inferi leaps out of the water. In the movie adaptation we do not see this but we do see the Inferi drag Harry into the lake and attempt to drown him.
- With the Horcrux placed on an island in the middle of the lake Lord Voldemort needed to have a way of getting to and from the island. There is an invisible boat which is anchored onto the cavern wall. The boat is only able to transport one wizard who has come of age. In the Half-Blood Prince, Harry is still in his sixth year of education and therefore is able to travel along with Dumbledore in the boat to the island. This again highlights another of Lord Voldemort's misjudgements as he did not expect such a young wizard to attempt to steal the Horcrux.
- The Horcrux is placed at the bottom of a basin on the island in the middle of the lake. The basin is filled with an enchanted potion which when drunk, causes the consumer to experience terrible things and causes almost an unbearable burning sensation within their body. After consuming all of the potion the person would be left dehydrated and would need water. Summoning water does not work and disappears almost immediately. This forces the drinker to collect water from the lake and disturbing the Inferi that lie beneath the surface. In the book and movie, Harry collects wtaer from the lake for Dumbledore and is then pulled in by the Inferi. Dumbledore then regains enough strength to cast a spell and repel the Inferi.
Harry and Dumbledore successfully retrieve what they believe to be the Horcrux, from the cave. At the end of the book it is revealed that the locket is actually a fake and a small note inside reads; "To the Dark Lord, I know I will be dead long before you read this but I want you to know that it was I who discovered your secret. I have stolen the real Horcrux and intend to destroy it as soon as I can. I face death in the hope that when you meet your match, you will be mortal once more. R.A.B." In the final book of the series, The Deathly Hallows, R.A.B is revealed to be Regulus Arcturus Black, the brother of Harry's godfather, Sirius Black.
 R.A.B, Kreacher and Lord Voldemort
Harry and Dumbledore were not the first pair to breach the cave in search of Lord Voldemort's Horcrux. Regulus Black, with the help of his house elf Kreacher, stole the original locket and took it back to the House of Black. After creating the Horcrux and setting up the enchantments to protect it, Voldemort asked Regulus Black is he could borrow Kreacher to test his enchantments. Kreacher travelled with Voldemort to the cave and inside, Kreacher was made to drink all of the potion so Voldemort could see the effects his enchantment would have on any thieves. Voldemort then left Kreacher on the island to be killed by the Inferi in the lake. However he forgot that house elves could apparate and disapparate within the cave and Kreacher was able to return home and tell his master, Regulus, what had happened. Regulus hated Lord Voldemort for this and upon hearing details from Kreacher, worked out that Voldemort had created a Horcrux. He made it his mission to retrieve and destroy the Horcrux with the help of Kreacher. The pair were able to retrieve the locket but were never able to destroy it. Eventually it was stolen from the House of Black by Mundungus Fletcher and passed on to Dolores Umbridge. We learn this in Harry Potter and the Deathly Hallows.
Harry Potter and the Half-Blood Prince - Inferi attack scene