"You know Voldemort's snake, Neville? He's got a huge snake... Calls it Nagini... It's got to be killed. Ron and Hermione know that, but just in case they — Just in case they're — busy — and you get the chance —"
Neville Longbottom: "Kill the snake?"
Harry Potter: "Kill the snake."
— Harry arranges for Neville Longbottom to kill Nagini,
“Nagini (d. 2 May, 1998) was a long, green female snake belonging to Lord Voldemort, with whom she had a special bond. She also became a Horcrux, after her master killed Bertha Jorkins in 1994. After Voldemort's initial downfall, he used Nagini's venom as one of the ingredients for a potion in order to regain strength, which eventually led to his rebirth. In 1995, she attacked Arthur Weasley, but he managed to survive. During the Second Wizarding War, she had to be destroyed in order for Voldemort to be finally defeated. Nagini was killed by Neville Longbottom with Godric Gryffindor's Sword in 1998, and was the last Horcrux to be destroyed.”
From Harry Potter WiKi