Little is known about who is considered on this list. However, it is certain that all the Dark Witches and Wizards on the list are immensely powerful and dangerous and considered among the greatest users of dark magic in history. Two wizards in particular set the benchmark in their infamy, Gellert Grindelwald and Lord Voldemort.
Gellert Grindelwald was number one of the list for the fanatical revolution he engineered against the European Ministries of Magic in his search for all three of the fabled Deathly Hallows, with which he intended to overthrow the International Statute of Wizarding Secrecy and establish a benevolent, hierarchal world order run by wizarding supremacists, with Muggles made subservient to witches and wizards and himself declared the invincible dictator of all time.
Born in 1883, Grindelwald grew up showing keen interest in the lore and mystique of magical artefacts, but was so fascinated with the Hallows' legend in the The Tales of Beedle the Bard to the point of adopting their runic symbol as his own personal emblem. He attended Durmstrang Institute for Magical Learning in 1893, where he excelled as the most brilliant student in the school's history, but was expelled at the end of his sixth year for conducting twisted experiments and thus continued his own magical education abroad.
In the summer of 1899, his historian great-aunt Bathilda Bagshot (the renowned author of A History of Magic) invited him to her home in Godric's Hollow, the British wizarding village which served as the burial place of the three brothers who originally owned the Hallows. During his stay, she introduced him to her brilliant yet like-minded young neighbour Albus Dumbledore, who was drawn to Grindelwald's ideas of greater power. Together, they planned on rebelling against Muggle authority in brutal ways that were (in their own words) "For the Greater Good". When Dumbledore's younger brother Aberforth found out, he confronted the duo and attempted to interfere with the conspiracy, but Grindelwald retaliated by lashing out with the Cruciatus Curse. Dumbledore intervened, sparking a three-way duel in which the brothers' magically unstable younger sister Ariana was killed and Grindelwald was forced to flee, never to be welcomed to Godric's Hollow again.
He managed to create Nurmengard fortress to hold his prisoners and his revolution killed many, leaving him to be much feared, making his position as the #1 most dangerous dark wizard well deserved. With the Elder Wand augmenting his already immense power, Grindelwald seemed to be unstoppable, with many who fought him being defeated and killed. As he continued to amass more power, public quickly began to plead for Dumbledore to fight him as he was the only one who could possibly be Grindelwald's superior. Although Dumbledore tried his best to resist it due to his fear of Grindelwald knowing who killed Ariana, eventually Grindelwald proved too powerful and dangerous and Dumbledore was finally forced to combat him in a duel so legendary it was noted to be the greatest wizarding duel of all time, and Dumbledore became world renowned for his feat. He was imprisoned in his own prison and lived there until he was murdered by a wizard more dangerous than himself, Lord Voldemort, dying trying to protect the Elder Wand from him as an attempt to achieve a degree of redemption.
Tom Riddle is widely considered the most evil wizard of all time, two and half decades after Grindelwald, for the two civil subversions he engineered against the British Ministry of Magic under the alias "Lord Voldemort", with which he intended to overthrow the Fundamental Laws of Magic and establish a sinister, tyrannical world order run by pure-blood supremacists, with Muggles and Muggle-borns exterminated and himself declared the immortal ruler of all time.
Born on 31 December, 1926, Riddle was conceived under the coercive effects of a Love Potion used by his mother Merope Riddle (née Gaunt), a pure-blood vagabond housewitch, to ensnare his father Tom Riddle Snr, a Muggle squirearch, into a loveless relationship. When the potion was allowed to wear off after three months of their marriage, Riddle Snr fled back to his ancestral manor, leaving Merope pregnant and destitute on the streets of London until she died in childbirth of a broken heart when sheltered in the nearby Wool's Orphanage. As a result, the orphaned Riddle was left to spent the first eleven years of his life in said orphanage, using his developing magic powers to control and intimidate his fellow orphans.
Riddle was brought into the wizarding world in the summer of 1938 by Albus Dumbledore, and started attending Hogwarts School of Witchcraft and Wizardry later that year, where he was sorted into Slytherin Hoiuse. Although his unhealthy mental state worsened as he began his quest for domination, he simultaneously got better at concealing it and used his good looks and charm to ingratiate himself with his teachers (with the notable exception of Dumbledore) and classmates. Many of them were taken in by his charisma, and by the start of his second year, Riddle was made leader of a like-minded gang of Slytherin boys known as the Knights of Walpurgis. The members of this gang, who all came from wealthy pure-blood families, were involved in a number of nasty incidents during their school years, though Riddle was careful to never become implicated in any wrongdoing and was generally held as a model student. Over time, Riddle grew obsessed with discovering his magical ancestry. He came to the conclusion that his father must have been a wizard because his mother suffered death, a mysterious and terrifying fate he assumed powerful witches and wizards could avoid. When he could find no trace of any Riddles with magical blood, he was forced to accept that his father was a "filthy Muggle" and that the magically powerful could indeed, die. Enraged by this discovery, he rearranged the letters of his own full name ("Tom Marvolo Riddle") to give himself a new one: "I Am Lord Voldemort", which he used among his friends.
Riddle finally found his magical ancestry in his fourth year when he used his middle name Marvolo (derived from his maternal grandfather Marvolo Gaunt) to discover the Gaunts and their descent from the Slytherin family. This explained that his exceptionally rare ability to speak Parseltongue made him the "Heir of Slytherin", rumoured to be the sole person who could open the legendary Chamber of Secrets. However, his most burning obsession remained the Chamber itself, the hidden entrance of which he spent the majority of his fifth year searching for despite most people insisting it was nothing but a myth. Finally, near the end of the year, he succeeded where countless others had failed by discovering the entrance in a girl's bathroom, which led down into the sewers beneath Hogwarts Lake. Entering the chamber, Riddle encountered the millennium-old Basilisk living inside and confirmed his connection to the historical dark wizard who bred it.
On 13 June, 1943, Riddle unleashed the beast into Hogwarts to continue Salazar Slytherin's "noble work" of purging the school of Muggle-born students. Several were petrified and one named Myrtle Warren was killed. However, this prompted Headmaster Armando Dippet and the Hogwarts Board of Governors to discuss closing the school in the interest of safety. Faced with returning to the Muggle orphanage he so despised, Riddle abandoned the Chamber of Secrets and framed half-giant third year Gryffindor Rubeus Hagrid and his pet acromantula Aragog for the crimes. As a result, Hagrid was expelled but offered to be trained as the school's Keeper of Keys and Grounds at Dumbledore's request, whereas Riddle was unjustly rewarded the Special Award for Services to the School. With his fear of death exacerbated by having stared it in the face, he used the murder of Myrtle to turn his school diary into his first Horcrux, the method to create which he feverishly researched in the Hogwarts Library's Restricted Section and learned from a copy of Secrets of the Darkest Art that was held there.
Around August, 1943, Riddle tracked down his Gaunt relatives to Little Hangleton, where his maternal uncle Morfin Gaunt gave him a biassed account of his father leaving his mother to die and abandoning him to the orphanage. In a desire for revenge, he entered the Riddle House and used the Killing Curse to kill his father alongside his Muggle grandparents, Thomas and Mary, then framed Gaunt for the crimes by placing him under the influence of a False Memory Charm. As a result, Gaunt boastfully confessed to killing his own in-laws and was sentenced to life imprisonment in the Dementor-infested wizard prison Azkaban (whereas the Riddles' caretaker Frank Bryce was suspected, though not prosecuted, of the murders within the Muggle community), while Riddle stole the Gaunts' signet ring (which containedthe Resurrection Stone) and started wearing it as a trophy. Shortly afterwards, he returned to Hogwarts for his sixth year and discussed the theoretical possibility of creating six Horcruxes (thus splitting the soul into seven pieces) with Professor Horace Slughorn, the Potions Master and his Head of House, who was taken aback by Riddle's interest in such Dark Magic. Although reluctant at first, Slughorn nonetheless gave him an overview of such an experiment with the promise of keeping their discussion between only them. This apparently gave Riddle what he wanted to hear, and he used the murder of his own father to turn the ring into his second Horcrux.
Riddle graduated from Hogwarts and quickly applied for the position of Defence Against the Dark Arts professor, which had just been vacated by Galatea Merrythought. Dumbledore argued against having him on the staff, so Dippet ended up cordially rejecting Riddle on the basis of being too young. However, he began developing an interest in the relics of the Hogwarts founders, and before leaving, he charmed the Grey Lady, the ghost of Ravenclaw House who was a member of the Ravenclaw family, to reveal where she hid Rowena Ravenclaw's diadem: a remote forest in Albania. So he travelled there and retrieved it from a hollow tree, using the murder of an Albanian peasant he encountered along the way to turn the diadem into his third Horcrux. When he returned to England, Riddle was offered several positions in the British Ministry of Magic, but turned them all down and began working at Borgin and Burkes, a dirty and disreputable Dark Arts shop in Knockturn Alley run by Caractacus Burke, which many believed was a waste of his talents. Tasked with smooth-talking people into parting with their possessions for far less than the actual cost, he used his position to learn more about the Dark Arts and form a one-sided friendship with Hepzibah Smith, a wealthy old antiques collector descended from the Hufflepuff family who was attracted to him.
His crimes were considered so evil and was feared so much that the majority of the wizarding world would not even speak his name, flinching if heard, and instead referring him as "You-Know-Who" and the likes. Many believe that Voldemort was not human enough to die after his first downfall, furthering his superiority as the most dangerous and depraved dark wizard, as unlike Grindelwald, he was unrepentant. Others chose to wallow in self-denial, and insisted he died, simply because they could not bare to confront the horrible fact he is still alive.
Voldemort managed to keep his activities hidden and graduated from with top grades, considered to be by many and by Albus Dumbledore himself to be the most brilliant student Hogwarts has ever seen. Voldemort preferred to operate in secret as opposed to Grindelwald's open revolution, to sow the seeds of distrust and keep the oppressed confused, and having studied arcane and obscure magic after graduation while keeping a low profile and experimenting so ill-advisedly with the boundaries of magic to gain the power that transcend what others believe to be "normal evil". Voldemort's first and final downfall at the hands of Harry Potter made the latter a world famous hero.