Emerald Potion

The Drink of Despair is a mysterious potion which induces fear, delirium, and extreme thirst. The drink cannot be penetrated by hand, vanished, parted, scooped up, siphoned away, Transfigured, Charmed, or made to change its nature in any way. It can only be drained away by drinking. In spite of all this, the potion can be magically refilled by a skilled spellcaster.

Lord Voldemort used this potion to protect Salazar Slytherin's Locket (one of his Horcruxes) within the Horcrux cave. This would provide a powerful defence, as one person cannot drink the entire content without collapsing from the terrible effects, and would require a second person to force feed the potion, which Voldemort considered impossible due to the enchanted boat is designed to carry only one adult Wizard (though Voldemort did not consider underage wizards or magical creatures such as house elves, presumably because he underestimated them).

History
When Lord Voldemort decided to use the cave as a location to guard his locket Horcrux, he brew this potion and placed it on the island in the middle of the subteranian lake. The potion was placed in basin and the locket Horcrux was put underneath. The potion was designed to mentally torture the drinker, give him/her intense stomach pains and make him/her dehydrated. It also caused the drinker to not be able to drink any water conjured magically. This then forced the drinker to drink from the lake, whose Inferi drowned anyone who touched the water.

In order to test the effectiveness of the potion, and that of the cave in general, he called upon one of his Death Eater's to lend him a House Elf. He didn't tell any of them what he was going to do with the house elf and none of his Death Eaters knew that he had created Horcruxes. He asked one of his Death Eater's, Regulus Black and Regulus gave him his elf, Kreacher. Regulus told Kreacher to return home when the task was done.

Voldemort tested all the defences including the potion, which as Voldemort hoped tortured Kreacher. Kreacher was then left to die on the Island after finishing the entire potion. Voldemort however didn't know that his anti-dissaparation jinx didn't work on House Elves, as he had underestimated them. This allowed Kreacher to return home and tell Regulus what had happened.

Regulus Black drinks the potion
Main Article: First Skirmish at the Horcrux Cave

Regulus was infuriated by Voldemort's treatment of Kreacher and sought to betray him and launch an attack. He also worked out, from various subtle hints given by Voldemort, that he had created Horcruxes, which angered Regulus even more. Regulus didn't tell anyone about his discovery, not even Kreacher and so his secret would die on his death. Regulus asked Kreacher to apparate him to the cave and to force feed him the potion when it got too much to bare, take the Locket and swap it for a duplicate one with a note to Voldemort, leave Regulus to die on the island and not tell anyone in his family what had happened. Kreacher was reluctant to do so but couldn't disobey a direct order. Regulus drank the potion and suffered from the effects and as Kreacher left, tried to drink from the water and was drowned by the Inferi. Kreacher never managed to destroy the Locket, and kept it at home until it was stolen by Mundungus Fletcher.

Albus Dumbledore and Harry Potter's attempt on the Locket
Main Article: Second Skirmish at the Horcrux Cave

Albus Dumbledore always suspected that Voldemort had created Horcruxes and his theory recieved evidence when Harry Potter destroyed Tom Riddle's Diary which had been used to open the Chamber of Secrets. In 1996, researching for Voldemort's ancestry he stumbled upon another Horcrux, Marvolo Gaunt's Ring. He destroyed the Horcrux but not before absent midently putting it on, thinking he would be able to see his dead family. When he put it on, he was cursed terribly. He managed to get back to Hogwarts in time to see, one of his employees, Severus Snape but the latter was only able to extend his prognosis for another year. Voldemort had also ordered, the teenager Draco Malfoy to kill Dumbledore, threatening to kill the boy's parents. Dumbledore then ordered Snape to kill him when the time was right.

Knowing that he only had one year left, and that Voldemort had put an assasination attempt for him, he decided to find out as much, and teach Harry Potter as much about Voldemort's Horcruxes so that Harry could finish off the task of destroying all of them when Dumbledore was dead. They discovered that he had made seven horcruxes and that they were mostly relics from the founders of Hogwarts.

Description
The Drink of Despair has an emerald green colour to it, and glows phosphorescently. As such, a green glow can be seen from the basin on the island in the middle of the lake from the far end of the cave. Even though Dumbledore said he could not change its nature in any way, Lord Voldemort turned the potion "crystal clear" to check the locket. It may be because he alone can alter the potion's characteristics to check the Horcrux, or because he only wanted to check on it rather than remove it from the basin. The former is most likely as it seems that Dumbledore could not change the colour to see if the horcrux was there, however Dumbledore might have been certain that the Horcrux was there, therefore feeling no need to check

Effects of the potion


"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 Mistress 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 defences.

While its effects are not completely known, the potion would seem to cause the drinker intense stomach pains, extreme thirst and cause them to see and believe terrible things, most likely their worst memories and fears. Having said that, the drinker always knows that they are reliving those experiences; not seeing them for the first time as they know that it is the potion causing them to suffer, otherwise they wouldn't ask to stop drinking.

Kreacher said (see quote above) that he saw 'terrible things' and that his 'insides burned'.

Albus Dumbledore's screams of "kill me" could either indicate a resurfacing of the memory of his sister's death (as in, "kill me instead") or a request to be delivered from the intense physical pain he was experiencing, or both. It also causes the drinker to become extremely thirsty and for any conjured water to vanish before reaching their mouths. This causes them to crawl to the water's edge in the Horcrux Cave and drink from the lake, activating the masses of inferi to rise out of the water and drag them down to drown; Dumbledore noted that this fearsome combination made it a powerful defence for the horcrux.

In 1998, while telling Aberforth Dumbledore about what happened when his brother drank the potion, Harry mentions that he believed that Dumbledore was reliving the 3-way duel that broke out amongst them and Grindelwald that resulted in their sister, Ariana's death. This was something that Dumbledore felt guilty about for the whole of his life after the event. While it was likely that this was what Dumbledore was seeing this duel and his sister's death, he did not mention out loud either events while he was drinking the potion.

Behind the scenes

 * In the movie, the Drink of Despair is a completely different colour, appearing as a murky black colour.
 * Also, in the book version, Dumbledore conjured a crystal cup out of thin air to drink the potion, while in the movie version, there was a seashell-shaped cup placed on top of the basin.
 * Though unnamed on the books, films, and video games, the track from the film's soundtrack which plays over the scene of Dumbledore consuming the potion is titled "The Drink of Despair." This would be an appropriate name for the potion, given the morbid effect that it has on its drinker.
 * Dumbledore's theory that the potion would not kill its drinker, but stop him in some way from reaching the Horcrux and possibly notify Voldemort (so that he could torture and interrogate the drinker) proved wrong; There was no alarm system as Voldemort was not notified when Regulus and Kreacher or when Albus Dumbledore and Harry entered the cave. The potion was also not immediately fatal, but was designed to make the drinker go to the lake and be drowned by the Inferi or die of thirst. It is unknown whether Dumbledore genuinely believed what he told Harry, or whether that was to persuade Harry to help him drink the potion.
 * Both human drinkers - Albus Dumbledore and Regulus Black were the only two people who independently discovered that Voldemort had created Horcruxes. (Harry, Ron and Hermione were told.) For Regulus Black, this was the first (and only) attempt at finding one of Voldemort's Horcrux, while Dumbledore had already found and destroyed the ring horcrux and was already dying from the ring's curse when he drank the potion. They were the only two drinkers who willingly drank the potion, knowing that they had to do so in order to obtain the Horcrux.
 * Both Albus Dumbledore and Regulus Black died shortly after drinking the potion (and knew that they were going to): Regulus Black presumably believed that he couldn't continue opposing Lord Voldemort and live, whereas Dumbledore only had a few months to live (due to being cursed by putting on the ring) and had ordered Severus Snape to give him a painless death. Regulus ordered Kreacher to leave him to die after he had drunk the potion and was taken by the Inferi while drinking from the lake to quench his thirst, while Dumbledore was killed by Severus Snape on Dumbledore's orders.
 * The torture inflicted by the potion may have contributed to Kreacher's bitter personality (along with Regulus's and Walburga's deaths, being unable to tell his family about Regulus's death, living alone with the Locket Horcrux and Walburga's portrait and Sirius Black's cruel treatment of him) and his unhealthy appearance (possibly along with age).
 * In LEGO Harry Potter: Years 5-7, the potion is kept in a water-cooler rather than a basin.

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 Half-Blood Prince (soundtrack)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * LEGO Harry Potter: Years 5-7