The seaside cave is a remote and magically protected location first discovered by a young Tom Riddle during a trip from Wool's Orphanage. It was here that he magically terrorized two other orphans, Amy Benson and Dennis Bishop, an early demonstration of his cruelty and control over his powers. The experience left the two children deeply traumatized. Years later, as Lord Voldemort, he returned to the cave and transformed it into the hiding place for one of his most precious objects: Salazar Slytherin's Locket, which he had turned into a Horcrux. He layered the cave's natural inaccessibility with powerful Dark Magic to create a formidable fortress. The entrance is a dark fissure in a sheer black cliff face, accessible only by swimming through the freezing sea. Inside, a damp antechamber leads to a solid rock wall that only permits passage after a blood payment is made. Beyond this barrier lies an immense, dark cavern containing a vast, silent, black lake. At the center of the lake is a small island of dark rock. To reach the island, a tiny, enchanted boat must be magically summoned from the depths of the lake. The boat is enchanted to carry only one wizard of age, forcing any companion to be a child or to trust the primary wizard completely. The lake itself is filled with Inferi—reanimated corpses—that lie dormant unless the water is touched or disturbed. On the central island rests a stone basin filled with a glowing, emerald-green potion, known as the Emerald Potion or Potion of Despair. Voldemort placed the locket Horcrux at the bottom of this basin. The potion cannot be charmed, vanished, or transfigured; it must be drunk in its entirety to access what lies beneath. Drinking it induces terrible psychological torment, causing the drinker to experience their worst memories and fears, and creates an agonizing, unquenchable thirst.
The seaside cave is the primary setting for a pivotal chapter in Harry Potter and the Half-Blood Prince. Albus Dumbledore, having discovered the probable location of a Horcrux, took Harry Potter there to retrieve and destroy it. To pass the blood-toll, Dumbledore cut his own hand, and to cross the lake, he had Harry accompany him in the boat as Harry was not yet of age. On the island, Dumbledore demonstrated immense courage by drinking the Potion of Despair himself to spare Harry the ordeal. The potion severely weakened him, leaving him vulnerable and delirious. After Harry retrieved the locket from the empty basin, Dumbledore's intense thirst led Harry to conjure water from the lake, which awakened the Inferi. The two were forced to fight off a horde of the creatures, with Dumbledore, despite his weakened state, conjuring a massive ring of fire to drive them back before they could Apparate away. This mission proved to be a tragic turning point. The locket they retrieved was a fake, replaced by Regulus Arcturus Black (R.A.B.). The effort and the potion's effects left Dumbledore grievously weakened just before their return to Hogwarts, where he was killed by Severus Snape atop the Astronomy Tower. In Harry Potter and the Deathly Hallows, the house-elf Kreacher recounts the full story of the cave. He explains how Lord Voldemort forced him to drink the potion as a test of the defenses. Later, a disillusioned Regulus Black returned with Kreacher, drank the potion himself, and ordered Kreacher to take the real locket and destroy it. Regulus was subsequently dragged to his death by the Inferi, becoming one of them.
In the film adaptation of Harry Potter and the Half-Blood Prince, the cave is visually depicted as a jagged, crystalline structure rising from the sea. The basin on the island is also shown as being made of crystal, a detail not specified in the book. The Inferi attack is a major visual effects sequence, portraying them as emaciated, grey-skinned humanoid figures. (film)