Crystal Cave

"An eerie sight met their eyes: they were standing on the edge of a great black lake, so vast that Harry could not make out the distant banks, in a cavern so high that the ceiling, too, was out of sight. A misty greenish light shone far away in what looked like the middle of the lake; it was reflected in the completely still water below. The greenish glow an the light from the two wands were the only things that broke the otherwise velvety blackness, though their rays did not penetrate as far as Harry would have expected. The darkness was somehow denser than normal darkness"

- Harry Potter and Albus Dumbledore as they go into the Horcrux cave

The Horcrux cave was the cave by the sea where Tom Riddle was taken as a child. As Lord Voldemort, he later used the cave to hide Salazar Slytherin's Locket, one of his Horcruxes. The cave was used again when Regulus Black stole the original locket and when Albus Dumbledore and Harry Potter went searching for it.

In his childhood, Tom Riddle was brought to a seaside site near this cave on holiday with his orphanage. He discovered this cave and terrorised two fellow orphans, Amy Benson and Dennis Bishop, there. He most likely magicked their way to the cave scaring the children. They never spoke of the incident. Without magic, entering the cave would require uncommonly skilled mountaineering skills, as boats cannot approach the cliffs.

Horcrux hiding place
"Harry turned his head to look at the greenish glow toward which the boat was still inexorably sailing. He could not pretend now that he was not scared. The great black lake, teeming with the dead…"

- Harry Potter in the cave with Albus Dumbledore searching for one of Voldemort's Horcruxes

Voldemort later decided to use this cave as a location for one of his Horcruxes (Salazar Slytherin's Locket). It was a very difficult place to find and even Dumbledore had to make a lot of research about Voldemort's past. Upon storing the locket in the cave, Voldemort set up a large number of complex enchantments, designed to keep others out but still allow him access to the Horcrux, should he desire to visit or remove it. As acknowledged by Dumbledore himself, the defenses that Voldemort had set up within were well-designed in that one person alone could not enter and leave it without assistance.


 * He placed a spell on the cave so that it was impossible to Apparate inside it (though ineffective against house-elf magic, as Voldemort underestimated them).


 * A smaller entrance was made: a door which required a blood sacrifice, a weakness payment, to open. According to Dumbledore, this was due to a common misconception by Voldemort that physical pain would weaken a person entering the chamber.


 * An Inferi-infested lake separated the entrance of the cave from the Horcrux. Any attempt to use a Summoning Charm to get the Horcrux would cause one or more Inferi to leap out of the water to block the spell's effect. If the water is disturbed, the Inferi would emerge and attempt to drag the intruder into the water to drown, where the intruder would become an Inferius upon death. The Inferi are frightened off by warmth and light.


 * The Horcrux was placed on an island in the middle of the lake. To get there, one had to find an invisible boat anchored to the cavern wall. The boat was enchanted so that only one wizard of age could be transported in it. This once again shows Voldemort's habit of underestimating his opponents, because he did not expect any underage wizard would attempt such feats.


 * The Horcrux itself was in a basin filled with a potion that could not be touched, vanished, transfigured, or charmed; the only way to dispose of it was to drink it. Once drunk, though, it caused the drinker to see "terrible things", to experience almost unbearable burning in one's insides, and to be dehydrated. If the drinker still had strength left after consuming the potion, conjured water would vanish on them and they would have to drink from the lake; upon disturbing the water an army of Inferi would rise from the lake and attempt to kill the interloper by dragging them under the water and drowning them.

Theft of the Horcrux
The first incursion upon the cave was by Regulus Black and Kreacher after Voldemort asked Regulus Black for a house elf. Regulus Black offered Kreacher and he was taken here. Kreacher was forced to drink the potion and Voldemort left, leaving him to get killed by the inferi. Kreacher disapparated back as Voldemort didn't remember or realise that house elves could apparate in and out. Kreacher told Regulus Black what had happened and Regulus started to hate Voldemort. Regulus worked out that Voldemort had made horcruxes and asked Kreacher to take him back. Regulus got the locket, which was later stolen from the Black family home.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter: A Pop-Up Book
 * LEGO Harry Potter: Years 5-7
 * Harry Potter for Kinect