Inaccessible except by magic, this cave complex on the coast was visited by young Tom Riddle while he was on an outing to the coast with children from the orphanage (circa 1930). While there he terrorized two of his classmates, Amy Benson and Dennis Bishop (HBP13). Many years later, he magically stored and protected one of his Horcruxes in an inner chamber of the cave system.
The cave had a large number of complex enchantments placed on it, some placed by Voldemort and some perhaps by earlier wizards, which were designed to keep others away from the Horcrux:
Once the enchantments were in place, Voldemort took Kreacher with him to the cave, brought him across the lake, and made the house-elf drink the potion. He then deposited his Horcrux in the basin, departed the island, and left Kreacher behind to die. Kreacher managed to escape, though, when he was summoned back home by Regulus and he Apparated out (DH10).
When Regulus learned what had happened, he was so infuriated he returned to the cave with Kreacher as well to steal the Horcrux; when he drank the potion himself, though, he ran to the lake to drink the water, and was dragged underneath and killed by the army of Inferi (DH10).
The cave then remained empty for sixteen years, until Harry and Dumbledore successfully retrieved what they thought was Slytherin’s locket (HBP26). Only when Harry looked at it later did he realized it was not the real locket but the one Regulus had left in place of the true Horcrux (HBP28).
- It was impossible to Apparate inside the cave.
- A smaller entrance cave, with steps leading in, had "known magic," including a concealed door that required blood to open. Dumbledore disliked this magic; in describing it he "sounded disdainful, even disappointed, as though Voldemort had fallen short of the standards Dumbledore expected" (HBP26).
- Inside the larger cave (with a lake), a Summoning Charm caused an Inferius to leap out of the water.
- A small boat was hidden beneath the water, invisible.
- The boat only allowed one wizard of age to ride in it at a time (at least, Dumbledore believed this was so).
- On the other side of the lake, the Horcrux sat in a basin full of potion:
- The potion cannot be touched, Vanished, Transfigured, or Charmed; it had to be drunk
- When the potion is drunk it caused almost unbearable pain and makes the drinker thirsty.
- After finishing the potion, the drinker is extremely weak
- Water cannot be Conjured on the island; the only way to obtain it is from the lake.
- When Harry touched the lake, an army of Inferi erupted from it to attack him.
The description of the sea cave, the dangerous bright green potion, and the monster-filled water is reminiscent of Circe Invidiosa (1892), John William Waterhouse's depiction of Circe poisoning the water in a sea cove to turn Scylla into a sea monster.