Nagini's venom

The venom produced from Lord Voldemort's snake, Nagini, is an extremely potent poison that prevents the wounds of the victim from healing. This would lead to severe blood loss that must be compensated with a Blood-Replenishing Potion, or the blood loss severity would turn fatal. There is an antidote to it, though apparently hard to find.

1994
In 1994, Death Eaters Peter Pettigrew milked the venom from Nagini's fangs and used it, along with unicorn blood, to concoct a dark potion, which allowed Voldemort's shattered soul to regain a rudimentary, but physical form. This form is very weak, and it greatly inhibits Voldemort's capabilities, but granted him the ability to hold a wand and enough energy to perform sufficient magic, until he could regain his true form. Until then, Voldemort required the potion every few hours to keep his crippled body alive, and Pettigrew had to continuously milk Nagini for the venom to keep feeding his master. It was for this reason that Voldemort refused to let Wormtail escape under the lie to find another wizard for the true rebirth ceremony, as Voldemort himself could not milk Nagini for the venom and concoct the potion.

1995
In 1995, when Nagini was sent by Voldemort to infiltrate the Department of Mysteries, she met up with Arthur Weasley, who was sent by Albus Dumbledore to guard the prophecy, on behalf of the Order of the Phoenix. Nagini, who sensed Arthur hiding under an invisibility cloak, bit him in his chest and injected him with her venom, preventing his wound from healing.

Arthur was sent to St Mungo's Hospital for Magical Maladies and Injuries, and was to take a Blood-Replenishing Potion every hour to counter the blood loss that would otherwise cost him his life, but was otherwise fine. When Arthur was persuaded by trainee Healer Augustus Pye to try the Muggle remedy stitches to seal the wound, the venom melted the stitches, rendering them ineffective. Eventually, Arthur managed to find an antidote to dispel the venom.

1998
During the Battle of Hogwarts in 1998, when Voldemort mistakenly assumed that Severus Snape was the master of the Elder Wand, he ordered Nagini to kill Severus. The great serpent bit him in the neck, and injecting him with the venom, preventing it from healing (even by magical means), thus leading to Snape's demise.

Behind the scenes

 * When Nagini bit Harry at Godric's Hollow while coming out of her disguise as Bathilda Bagshot, it is unknown how dittany was capable of stopping the bleeding, when Arthur and Snape both bled unstoppably, with Snape actually dying from it. Lily Potter's love protection may immunize Harry from the venom. Alternately, Nagini can possibly choose whether or not to bite using her fangs, as many real life snakes can, thus deciding whether or not to apply venom. Another possibility is that Horcruxes cannot hurt each other.

Appearance

 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 2