Among Black's many magical talents was his ability to transform into a dog, marking him as an Animagus. Animagi are supposed to register their ability with the Ministry of Magic, something Black never got around to doing before being imprisoned in Azkaban.
Sirius was portrayed by Gary Oldman in the films.
Sirius was the last remaining heir of the House of Black, a once-proud family of Pureblood wizards. However, Sirius strongly disagreed with his relatives' views on Pureblood superiority and was looked upon with contempt, even downright hatred, by the rest of the family. While the rest of his relatives were proud members of Slytherin House while attending school at Hogwarts, Sirius broke away from tradition by requesting the Sorting Hat to place him in Gryffindor.
During his time at Hogwarts, Sirius befriended James Potter, Remus Lupin and Peter Pettigrew. The four boys became known as the Marauders and were regarded with mixed opinions by the students and faculty. While no one could deny that they each possessed great magical talent, Sirius and his friends were also notorious troublemakers and occasionally even bullies; they frequently victimised the young Severus Snape which would lead to Snape harbouring a bitter, life-long grudge against them.
First Wizarding WarEdit
After graduating from Hogwarts, James Potter married Lily Evans and Sirius attended the wedding as best man. When their son Harry was born, James and Lily named Sirius godfather, thus designating him as Harry's guardian in the event of their deaths. He also gave Harry his first broom at the age of one as a birthday present, as stated by Lily in a letter found by Harry a number of years later.
As Lord Voldemort gathered support for his revolution against the Ministry of Magic, Sirius found himself afraid and confused, uncertain of who he could trust as the Death Eaters' influence spread. He confided in his fellow Marauder Peter Pettigrew, whom he trusted implicitly. This would be a decision Sirius would come to regret for the rest of his life as Pettigrew was secretly in league with the Dark Lord, telling him everything he had learned from Black, including the location of the Potter family when Voldemort became aware of the prophecy of the one who could best him. Because of Pettigrew's betrayal, the Potters were slain by Voldemort and Black lost his closest friends.
Sirius swore revenge against Pettigrew for his involvement in the death of the Potters, confronting him in the small village of Godric's Hollow where the Potters resided at the time of Voldemort's attack. Black would be cheated his opportunity for retribution when Pettigrew called out to the townspeople that Black had killed the Potters immediately before causing a magical explosion. The explosion allowed Pettigrew to fake his death and frame Sirius for his murder and the murder of several innocent bystanders who were present. Pettigrew escaped and Sirius was arrested by Aurors, later sentenced without trial by the Ministry of Magic to an indefinite period in the wizard prison of Azkaban.
Escape from AzkabanEdit
Sirius' escape was legendary to say the least. In 1993, he became the first known person to ever escape from the wizard prison of Azkaban. After several trying years in Azkaban, Sirius regained enough strength to transform into his Animagus form. Since Dementors have difficulty sensing the less complex emotions of animals, he was able to remain relatively unaffected as a dog. After receiving an issue of the Daily Prophet from Cornelius Fudge during an inspection, Sirius discovered that Pettigrew was still alive and in hiding in his Animagus form as Ron Weasley's pet rat. Filled with the desire for revenge, Sirius took his Animagus form and, thinned from malnourishment, was able to slip through the bars of his cell and past the Dementors. He swam across the North Sea to freedom, though the journey nearly killed him. He made his way to Little Whinging to catch a glimpse of his godson as a teenager, where his appearance startled Harry before the Knight Bus turned up. As news of Sirius muttering "he's at Hogwarts" in his sleep, and subsequently escaping, was released, the Wizarding community went on high alert, believing that he planned to murder Harry in a mad attempt to bring Lord Voldemort back.
Sirius took refuge around Hogsmeade and the Forbidden Forest, where he was spotted a few times and mistaken for the Grim. He made the acquaintance of Hermione Granger's cat Crookshanks, who had recognised that Sirius wasn't actually a dog, and who had also recognised Peter for what he was. Crookshanks attempted to bring Peter to Sirius, but Ron was very protective of his so-called pet. Half-crazed and desperate, Sirius sneaked into Hogwarts through the old passageway from the Shrieking Shack and on one occasion slashed the Fat Lady's portrait when she refused him entrance to Gryffindor Tower. Later, he came to view the Quidditch match between Gryffindor and Hufflepuff, where he witnessed Harry's excellent flying skills, which mirrored James'. Eventually, he was able to enter the Gryffindor common room with a list of passwords Crookshanks had stolen from Neville Longbottom and shredded Ron’s bed curtains in his search for "Scabbers." Peter was nowhere to be found since Sirius's escape had prompted him to once again fake his own death and hide.
In June, Sirius caught Ron carrying Scabbers, after the rat had been discovered hiding in Rubeus Hagrid's cabin, and dragged both boy and rat through the tunnel under the Whomping Willow to the Shrieking Shack. Harry and Hermione Granger followed, the former intent on confronting Sirius about the supposed betrayal of his parents, despite the obvious differences in their ages and skill levels; Sirius' weakened state (due to being on the run) at that time led to the boy easily, and wandlessly, overpowering the older wizard despite Crookshanks's attempts to aid the Animagus. Once Harry had Sirius at his mercy, Remus Lupin, who had seen Peter on the confiscated Marauder's Map, disarmed Harry and warmly greeted his old friend. Together, Lupin and Sirius revealed Peter's true form. Both wanted to kill their former friend for betraying James and Lily, but Harry prevented them from doing so, believing that it would be better to force Pettigrew to face justice at the hands of the Dementors. That, and he didn't want his father's friends to be murderers.
As they made their way back to the castle, Sirius hesitantly asked Harry if he would like to live with him, thinking Harry would not want to live with the aunt and uncle who had raised him; Harry enthusiastically agreed, thrilled at the idea of leaving the Dursleys and finally having a real home. Unfortunately, soon after they left the Whomping Willow, Lupin transformed under the full moon and, having forgotten to take the Wolfsbane Potion in the wake of discovering Pettigrew, was uncontrollable. In order to protect Harry, Ron, and Hermione from Lupin, Sirius turned back into a dog, giving Pettigrew an opening to escape and flee. Sirius was weakened following his encounter with Lupin's werewolf form and could not protect himself when the Dementors guarding Hogwarts arrived. Harry, who had followed Sirius and found him lying unconscious and surrounded by the hooded creatures, attempted to perform the Patronus Charm but to no avail. The Dementors nearly succeeded in sucking out Sirius' soul until a powerful Patronus resembling a stag warded them off.
Sirius was briefly re-captured by Snape, and was sentenced to the Dementor's Kiss, a fate worse than death. Miraculously, and by Albus Dumbledore's suggestion, Harry and Hermione used Hermione's Time-Turner to help Sirius escape; as Dumbledore had also hinted that "more than one innocent life" could be saved that night, they rescued the hippogriff Buckbeak from execution, thus granting Sirius a form of travel. Sirius was once again a wanted man, although alive and with his soul intact. Soon after his escape, he sent Harry, Ron and Hermione a letter via a tiny, hyperactive owl, explaining that it was he who sent Harry the Firebolt for Christmas, and giving Harry permission, as his godfather, to go to Hogsmeade. He also apologised to Ron for the loss of his pet, asking him to accept the owl in "Scabbers" place.
Order of the PhoenixEdit
Sirius spent over a year after his escape in hiding outside of Europe, beyond the jurisdiction of the Ministry of Magic, occasionally maintaining contact with Harry in secret. When Lord Voldemort was restored to his physical form, Dumbledore reinstated the Order of the Phoenix, sending Sirius to gather the old crowd and inform them of the situation. Sirius donated his family home at 12 Grimmauld Place in London as Order Headquarters. Unfortunately, as he was still wanted by the authorities, Sirius could never safely leave the house and became quite bitter over his perceived "uselessness". When he briefly left the house to see Harry to the Hogwarts Express, Draco Malfoy nastily complimented Harry on his "pet dog," which, coupled with comments Malfoy made on the train, suggested Sirius' cover was blown. Harry and Sirius stayed in touch during Harry's school year via owls and the Floo Network, though Sirius' presence was nearly discovered by Dolores Umbridge during her dictatorial managing of Hogwarts that year.
Sirius acted as more of a brother than father figure to Harry throughout his hardships with Umbridge, encouraging him to oppose her reforms and strongly approving of Harry's secret defensive tutorial group for students, Dumbledore's Army. His vigorous support of Dumbledore's Army worried Hermione, who thought that Sirius was attempting to live vicariously through them. He also willingly answered all of Harry's questions about the Order and Voldemort, though most of Harry's mentors felt Harry was too young to handle the burden of the truth. He also urged Harry to contact him if Professor Snape gave him a hard time during their Occlumency lessons.
During his time confined to the Order's headquarters, Sirius began to let himself go. When Harry and the Weasleys arrived at Grimmauld Place just after Arthur was attacked by Nagini, Sirius was unshaven and still in his day clothes late at night. He also seemed to have taken to drinking, as he had a "Mundungus-like whiff of stale drink about him". However, Sirius made a complete turn-around when the Weasley family and Harry decided to stay at Grimmauld Place over the Christmas holidays for its proximity to St. Mungo's, and at one point was heard singing "God Rest Ye, Merry Hippogriffs" at the top of his lungs.
Voldemort used Legilimency to implant a false vision into Harry's mind that Sirius was being tortured in the Department of Mysteries, convincing Harry that Sirius had in fact been captured. After giving Severus Snape a cryptic message informing him of the situation and then ditching Umbridge in the Forbidden Forest, Harry, along with Ron, Hermione and fellow D.A. members Ginny Weasley, Neville Longbottom and Luna Lovegood, flew to London via Thestral. The students made their way through the deserted Ministry of Magic and gained access to the Department of Mysteries, only to be ambushed by Death Eaters.
It was Snape who ultimately saved their lives by alerting the Order to what Harry had seen; Sirius, Kingsley Shacklebolt, Nymphadora Tonks, Remus Lupin, Alastor Moody, and eventually Albus Dumbledore, arrived at the Ministry and began battling the Death Eaters. Tragically, however, during a frenzied duel with his hated cousin Bellatrix, Sirius was struck with a spell, causing him to fall through the Veil in the Death Chamber to his death. It would be years before Harry could fully come to terms with the loss of his godfather, who, as Dumbledore flatly stated, was the "closest thing to a parent" Harry had ever known.