Lord Voldemort is a sobriquet for Tom Marvolo Riddle, a character and the main antagonist in J. K. Rowling's series of Harry Potter novels. The character first appeared in Harry Potter and the Philosopher's Stone, which was published in 1997, and returned either in person or in flashbacks in each book and its film adaptation in the series except the third, Harry Potter and the Prisoner of Azkaban, in which he is only mentioned.

Voldemort is the archenemy of Harry Potter, who according to a prophecy has the power to vanquish the Dark Lord. He attempts to murder the boy, but instead kills his parents, Lily and James Potter, and leaves Harry with a scar on his forehead in the shape of a lightning bolt. Nearly every witch or wizard dares not utter his name and refers to him instead with such monikers as You-Know-Who, He Who Must Not Be Named, or the Dark Lord. Voldemort's obsession with blood purity signifies his aim to rid the wizarding world of Muggle (non-magical) heritage and to conquer both worlds, Muggle and wizarding, to achieve pure-blood dominance. Through his mother's family, he is the last descendant of the wizard Salazar Slytherin, one of the four founders of Hogwarts School of Witchcraft and Wizardry. He is the leader of the Death Eaters, a group of evil wizards and witches dedicated to ridding the Wizarding World of Muggles and establishing Voldemort as its supreme ruler.