Basilisk venom

Basilisk venom is an extremely poisonous substance that only has one cure - phoenix tears. It can be used to destroy horcruxes.

In Harry Potter and the Chamber of secrets, when Harry stabs the roof of the basilisk's mouth, and ends up getting a basilisk fang stuck in his arm. Later Fawkes comes in and heals his arm. When Harry had defeated the basilisk, he stabbed Tom Riddle's diary with the fang, destroying that horcrux beyond magical repair. After Harry had stabbed the basilisk with it, the sword of Godric Gryffindor was impregnated with basilisk venom.

In Harry Potter and the deathly hallows, during the second battle of Hogwarts, Ron and Hermione went down to the chamber of secrets and destroyed Helga Hufflepuff's cup with a basilisk fang.