The main antagonist throughout the Harry Potter series is Lord Voldemort. Born Tom Marvolo Riddle, he becomes a powerful dark wizard devoted to conquering the wizarding world, enforcing blood-purity ideology, and defeating the prophecy that links his fate to Harry Potter's.
Voldemort first appears in Harry Potter and the Philosopher's Stone, published in 1997, although he initially exists in a weakened, dependent form. His attempt to kill the infant Harry fails when the curse rebounds, leaving Harry with his famous lightning-shaped scar and Voldemort without a physical body. Across the series, he works through servants called Death Eaters and repeatedly seeks ways to restore his power.
Bellatrix Lestrange is one of his most devoted followers, while Snape's role is deliberately complicated and Grindelwald belongs primarily to an earlier generation of dark wizards. Voldemort is also called You-Know-Who and He-Who-Must-Not-Be-Named. His birth name, Tom Riddle, remains important because “Lord Voldemort” is derived from it.