Salazar Slytherin's Basilisk vs. Fawkes the Phoenix.
Basilisk venom is an extremely poisonous and corrosive substance generated by the said species.
Basilisk venom is so powerful that it can kill an infected person within minutes, making the person drowsy and blurry-visioned before they die, and is apparently extremely long-lasting, seeing as Hermione Granger was able use the venom from the fang of a Basilisk that was dead for nearly five years. The venom is one of the few substances known to be capable of destroying Horcruxes. It only has one known cure: phoenix tears, which happens to be very rare, increasing the venom's deadliness.
The effect of the venom
History
1993
In 1993, Harry Potter stabbed the roof of the Basilisk's mouth with Godric Gryffindor's Sword, and ended up getting a fang stuck in his arm. Fawkes then healed his arm using his tears. When Harry had defeated the Basilisk, he stabbed Tom Riddle's Diary with the fang, destroying that Horcrux beyond magical repair. Because Harry had stabbed the Basilisk with it, the sword was imbued with Basilisk venom, and was therefore able to destroy Horcruxes as well.
1996
Albus Dumbledore used Godric Gryffindor's Sword to destroy Marvolo Gaunt's Ring.
1997
Ron Weasley used Godric Gryffindor's Sword to destroy Salazar Slytherin's Locket.
1998
In 1998, during the Battle of Hogwarts, Ron and Hermione went down to the Chamber of Secrets and destroyed Helga Hufflepuff's Cup with a Basilisk fang. Neville Longbottom used Godric Gryffindor's Sword to kill Nagini.