He pulled it out of the Sorting Hat.
Harry Potter told Neville Longbottom that Nagini had to die so Neville Longbottom decapitated her.
If you're talking about Nagini then he cuts her head off with the Sword of Gryffindor. :)
Neville Longbottom beheads her, with the Sword of Griffindor.
Outside Hogwarts castle.
Neville Longbottom killed Nagani with the sword of Gryffindor.
Neville Longbottom beheaded Nagini, the snake.
Harry kills the basilisk and Neville kills nagini
Nagini is killed by Neville Longbottom using Gryffindor's Sword, which is an object that can destroy Horcruxes due to the fact that it had been impregnated with Basilisk venom.
Neville Longbottom beheaded Nagini
Neville Longbottom destroys the sixth Horcrux, Nagini, Voldemort's snake, with the Sword of Gryffindor during the Battle of Hogwarts.
Neville Longbottom chops off her head using the Sword of Godrick Griffindor during the Battle of Hogwarts. He takes the sword from the sorting hat that vokdemort placed on his head, and freed himself from the body binding curse.
Voldermort's snake, Nagini, gets killed by Neville Longbottom. In the Deathly Hallows Part 2, Nagini almost attacks Ron Weasley and Hermione Granger, but Neville kills it with the Sword Of Gryffindor.