A Maledictus is a female individual who carries a blood curse that eventually turns her into a beast permanently. The curse is carried from birth and passed down from mother to daughter. The beast ultimately transformed into can vary based on the curse. Before their permanent transformation, they have the ability to change shape at will, which becomes gradually uncontrollable.
Nagini was a female Maledictus cursed to transform into a snake. Originating possibly in Indonesia, by 1927 she was a performer at the Circus Arcanus where she met Credence Barebone and had the ability then to transform at will.
Nagini was seemingly trapped in snake form and belonged to Lord Voldemort, with whom she had a special bond largely due to becoming a Horcrux, after her master had killed Bertha Jorkins.
After Lord Voldemort's initial downfall, he used Nagini's venom as one of the ingredients for a Potion to regain strength, which eventually led to his rebirth. Later in the same year, she attacked Arthur Weasley but he managed to survive. During the Second Wizarding War, she had to be destroyed for Lord Voldemort to finally be defeated.
Nagini was killed by Neville Longbottom with Godric Gryffindor's Sword and was the last Horcrux to be destroyed after Rowena Ravenclaw's diadem.
After he discovered that Harry was searching for his Horcruxes, Lord Voldemort placed Nagini into a Protective Magical Cage to prevent her from being killed.
Shortly before the one-hour armistice during the Battle of Hogwarts, Lord Voldemort used Nagini to murder Severus Snape in the Shrieking Shack by expanding the cage over and on top of him.
When Harry Potter was apparently killed by Lord Voldemort, Nagini was released from the protective enchantment as he believed there to be no more threats to her life, and she was draped around Lord Voldemort's shoulders during the Death Eaters' victory march back to Hogwarts.
After Neville Longbottom had openly defied him, Lord Voldemort punished him by forcing the Sorting Hat onto his head and setting it on fire. The Death Eaters were then attacked, and during the ensuing battle, Neville Longbottom pulled out the Sword of Gryffindor from the Hat, and, after he swung the blade in one upward stroke, beheaded Nagini, whose head spun high into the air as her body slumped onto the ground, while Lord Voldemort screamed in rage.
With Nagini's death, Lord Voldemort's final Horcrux was destroyed, and thus stripping him of his immortality. He furiously tried to kill Neville Longbottom for it, but was stopped by Harry Potter. Lord Voldemort himself finally died when his Killing Curse rebounded onto himself once again.