The Horcrux Cave is a natural sea cave that a young Tom Riddle visited during an outing from his orphanage. It was here that he first began to explore his powers in a cruel and domineering fashion, magically terrorizing two other children, Dennis Bishop and Amy Benson, within its dark confines. The experience left the other children traumatized, and it became a place of private power and significance for Riddle. Years later, as Lord Voldemort, he chose this same cave as the hiding place for one of his most precious possessions: Slytherin's Locket, which he had transformed into a Horcrux. Believing no one could penetrate its defences, he wove a complex and powerful web of Dark Magic to protect it. The entrance to the cave is a dark fissure in a massive black cliff face, accessible only by sea. A short swim from a large offshore rock leads to the entrance. Inside, a dark, dripping passage leads to a solid rock wall. This wall serves as a magical door, which only opens when a blood sacrifice is made upon its surface. Beyond the wall lies a vast, pitch-black cavern containing a large, eerie lake. The only light emanates from a strange greenish glow in the center of the lake, where a small island of black rock is situated. Hidden just beneath the water's surface is a small, enchanted boat, tethered by an invisible chain. The boat is magically enchanted to carry only one fully-grown wizard across the lake at a time, preventing a would-be thief from bringing a powerful accomplice. The lake itself is filled with an army of Inferi—reanimated corpses of wizards and Muggles killed by Voldemort—that serve as the primary guardians of the Horcrux. They lie dormant unless the surface of the lake is disturbed or an intruder attempts to steal from the central island. On the island, a stone basin rests, filled to the brim with a poisonous, emerald-green liquid known as the Potion of Despair. This potion cannot be vanished, parted, or scooped out; it must be drunk in its entirety to reveal the Horcrux lying at the bottom. The potion causes the drinker to experience their worst memories and profound despair, leaving them physically and mentally incapacitated.
The Horcrux Cave is a pivotal location in Harry Potter and the Half-Blood Prince. Albus Dumbledore, having discovered the cave's significance through a memory in the Pensieve, takes Harry Potter there to retrieve what he believes is Slytherin's Locket. Their journey through the cave's defences serves as a stark demonstration of the depths of Voldemort's cruelty and paranoia. Dumbledore makes the blood sacrifice to open the magical wall and uses his knowledge of Voldemort's arrogance to find the hidden boat. Upon reaching the island, Dumbledore drinks the Potion of Despair himself, enduring immense suffering to protect Harry. This act severely weakens him just before their return to Hogwarts. After retrieving the locket from the basin, a thirsty Dumbledore asks for water. Harry's attempt to summon water from the lake awakens the Inferi, who drag Harry into the freezing depths. A barely-conscious Dumbledore musters his remaining strength to conjure a massive ring of fire, driving the Inferi back and saving them both. The greatest twist occurs when they discover the locket is a fake. Inside is a note from a mysterious individual identified only by the initials R.A.B., revealing that the real Horcrux has already been stolen with the intent of destroying it. This discovery proves that Voldemort's immortality was not as secure as he believed and that someone else had uncovered his secret long before Harry and Dumbledore. In Harry Potter and the Deathly Hallows, it is revealed that R.A.B. was Regulus Arcturus Black. His House-elf, Kreacher, recounts how Voldemort had forced him to drink the Potion of Despair when first placing the Horcrux. Regulus, upon learning of Voldemort's actions, returned to the cave with Kreacher, drank the potion himself, and ordered Kreacher to take the real locket and destroy it. Regulus was subsequently killed by the Inferi, sacrificing his life in his rebellion against the Dark Lord.
In the film adaptation of Harry Potter and the Half-Blood Prince, the cave is depicted as being filled with jagged, crystalline structures, a detail not specified in the novel. The confrontation with the Inferi is a major set piece, showcasing a dramatic visual effects sequence where Dumbledore conjures a powerful vortex of fire to repel them. (film)