Lord Voldemort

[close] Tom Marvolo Riddle was born to a pureblood mother and a muggle father in an orphanage in England, and his mother died shortly afterward. He uses his magical powers (including being a parseltongue) to torture the other children in the orphanage. When he is eleven, Albus Dumbledore informs him that he is a wizard, and at Hogwarts is a star student and forms a circle of friends who later all become the first Death Eaters. Riddle also uses the alias "Voldemort", taken from his name. This name, which is derived (in real life) from the French for "flight from death" shows that Voldemort is terrified of the concept of Death, and attempts to immortalize himself by splitting his soul and hiding parts in Horcruxes.
After Hogwarts, Voldemort established the Death Eaters and starts to change his appearance. He gains power gradually and by the time of James Potter and Sirius Black is a big threat, though fails to penetrate Hogwarts because of his nemesis, Dumbledore. He hears of the prophecy dictating a child who will be as powerful as he is from Snape, although he doesn't hear the whole thing and so thinks he can get rid of the baby immediately. Two children could have been the one in the prophecy: Harry Potter or Neville Longbottom. Riddle chose the half-blood, Potter, seeing himself in the babe. However Voldemort's spell backfires and he disappears, but doesn't die.
Riddle appears in the first two Harry Potter books, and "comes back" in the end of the fourth, and it is revealed to the wizarding world that he has in the end of the fifth. He seizes control of the Ministry of Magic in the beginning of the seventh book and constantly attempts to kill Harry Potter. He ends up using an unsuccessful killing curse on Harry during the Battle for Hogwarts, and then enters the school thinking himself triumph, though Harry then reveals himself to be alive. After a showdown in the great hall, surrounded by all the other participants of the battle, his Killing Curse backfires on Harry's Disarming Spell and Riddle dies.
Riddle is described by J. K. Rowling as being a "psychopath", and she compared him to Hitler.