"Only by drinking it can I empty the basin and see what lies in its depths."
-- Albus Dumbledore in the Sea Cave (HBP26)
Voldemort used a basin filled with emerald green potion in the sea cave to protect the Horcrux made from Salazar Slytherin’s locket (HBP26).
References from the canon
- Described as a "phosphorescent emerald liquid, designed to prevent the drinker from finishing it" (Pm)
- Voldemort put the basin of potion in the sea cave because that is where he terrorized two children from his orphanage as a child (HBP13).
- Dumbledore knew the potion would work slowly but would also keep him from taking the Horcrux out of the basin. He warned Harry to make him keep drinking it because "might paralyze me, cause me to forget what I am here for, create so much pain I am distracted, or render me incapable in some other way" (HBP26). And indeed, Harry had to force Dumbledore to keep drinking the potion.
- Dumbledore seemed to have terrible nightmares or visions, and was almost too weak to stand up after drinking the potion.
- Kreacher said that when Voldemort forced him to drink the emerald potion his "insides burned" and he saw "terrible things" (DH10).
- Regulus Black courageously decided to drink the potion so he could leave behind the fake Horcrux locket and the letter to Voldemort signed "R.A.B." but he did not survive as long as Dumbledore. He was dragged under water by the Inferi and died there, while Kreacher took the real Horcrux back to Grimmauld Place (DH10). Mundungus Fletcher later stole the Horcrux locket intending to sell it, but Dolores Umbridge confiscated it in Diagon Alley (DH11). Ron eventually destroyed it with the Sword of Gryffindor, sent by Snape (DH19).
- Sirius Black underestimated his house-elf Kreacher, and he also underestimated his brother Regulus, never suspecting that he died while doing something so courageous. Sirius told Harry that Regulus "...was murdered by Voldemort. Or on Voldemort's orders, more likely; I doubt Regulus was ever important enough to be killed by Voldemort in person ...he got in so far, then panicked about what he was being asked to do and tried to back out." (OP6). No, not exactly.
The Cave Potion has no name in the canon, and the only thing we know are the effects and the color, which is emerald green. Of course Salazar Slytherin's locket had a letter "S" formed in emeralds, and green is the color of Slytherin House, so we can guess Voldemort was trying to be symbolic.
Fan Theory: Snape told Umbridge "most venoms act too fast to give the victim much time for truth-telling" (OP32). But Dumbledore said Voldemort's Cave Potion was meant to kill the victim slowly, and it turns out there is a poison well-known from ancient times that kills slowly with some of the same side-effects seen with Dumbledore and Kreacher, and that is Poison Hemlock or Conium maculatum. The lethal effects of this plant are well-known, and has been used over the centuries to kill convicted prisoners, such as the famous Greek philosopher Socrates, who drank hemlock mixed with water and died a slow death.
The conium name derives from the Greek konas, meaning “vertigo” or “whirl”—both symptoms of the plant’s intoxicating poison. ...This poison disrupts the central nervous system—a small dose can cause respiratory collapse. Death can result from blockage of the neuromuscular junction caused by coniine. In practice, it eventually stops your ability to breathe, causing you to suffocate. This won’t happen right off, and drinking its tea only makes you feel drunk at first. It may take from 48 to 72 hours for the full toxic effects to manifest. Source: Slate
In the canon, the Doxycide potion contains not only Hemlock essence from poison hemlock but also Cowbane, known as "water hemlock" (Pm). While Doxycide is a black liquid, Hemlock essence is depicted on Pottermore as whitish-gray, while Cowbane has the same emerald green color as Voldemort's Cave Potion. It's possible Voldemort mixed them both together to make a super-poison. All of this is speculation, but using Doxycide ingredients is like using Pesticide on other people, and it fits with the Dark Lord's evil nature since he thought of others as bugs to be exterminated, and the sea cave as a trap.