Horcrux potion

When Lord Voldemort hid a locket he had turned into a Horcrux inside a cave that he had visited as a boy, he set in place many enchantments that would protect it, including this potion.

It was described as giving off a misty greenish light that could be seen even at a distance. The potion was capable of resisting any attempt at removing it, whether physically or magically, other than drinking it. Voldemort, however, was able to turn the potion clear in order to check the status of the Horcrux within.

"Kreacher drank, and as he drank, he saw terrible things... Kreacher's insides burned... Kreacher cried for Master Regulus to save him, he cried for his Mistriss Black, but the Dark Lord only laughed... He made Kreacher drink all the potion... He dropped a locket into the empty basin... He filled it with more potion..."

- Kreacher describing when Voldemort forced him to test the potion's defenses.

The potion was capable of severely weakening the drinker, as well as inflicting both physical pain and mental anguish. Albus Dumbledore was apparently forced to relive the moment when his friend Gellert Grindelwald attacked his brother Aberforth, and the subsequent duel that resulted in his sister Ariana's death. The house elf Kreacher stated that he "saw terrible things."

The potion also caused a desperate thirst in its drinker. However, any attempts to summon water magically, such as an Aguamenti charm, would be ineffective. Whether this was an effect of the potion itself or some other enchantment is unknown. The unfortunate victim would be forced to get water from the only other available source; the Inferi-infested lake. Although the potion itself would not kill its victim, the results would usually be just as fatal.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows