Voldemort threatened the entire Wizarding World for decades over his unnaturally long life. The overarching villain in the Harry Potter movie series, fans loved to hate the Dark Lord over the course of the story as he returned from the dead, raised an army of Death Eaters, and was responsible for the deaths of many favored characters from the universe.
As the unofficial "leader" of the Wizarding World's unsavory characters, Voldemort amassed an army of Death Eaters, followers, and even the Dementors from Azkaban. Having stood guard in Azkaban to feed on the memories of incarcerated wizards, these creatures eventually revolted, leaving Azkaban and joining Voldemort's dark army. As viewers of Harry Potter will know, Voldemort was eventually defeated by the titular character during the events of Harry Potter and the Deathly Hallows, even if it was slightly different in the Harry Potter movie version than the books. So what happened to Azkaban after that?
What Is Azkaban?
Azkaban was a fortress prison in the North Sea that held criminal wizards, shielding them from the outside world. Azkaban prison was created in 1718 to house wizards that were deemed a danger to society. Over the course of the 18th century, rules were added by the Ministry of Magic that any usage of the three Unforgivable Curses (Avada Kedavra, Crucio, and/or Imperio) would result in a life imprisonment at Azkaban.
The Dementors; hollow, magic-inhibiting creatures that fed on the memories of incarcerated wizards, stood guard over the prison, ensuring that none of the prisoners had the strength to escape, nor any happy memories to cling onto throughout their sentence. Occasionally, the Dementors would take their feeding too far, draining a wizard of any and all happy memories and draining their very soul, leaving them an empty shell destined never to be their usual self again. Sometimes, Dementors would attack Muggles, but only on rare occasions.
As the prison was intended to house criminals who posed a danger to society, it had natural defenses to prevent them from escaping. Wands were weighted and handed in at the security entrance, so visitors could not pass a wand to a criminal and allow them to try and escape. There was an Anti-Apparition charm to ensure that wizards couldn't apparate in or out of the prison, since some powerful wizards could do this even without a wand. Furthermore, broomsticks and any other travel methods for wizards were restricted and held at the security entrance.
Before the mass outbreak when Death Eaters and Dementors left to join Voldemort's army, only two wizards had ever managed to escape Azkaban. The escapees were Barty Crouch Jr, who posed as Alastar Moody during Harry Potter and the Goblet of Fire, and Sirius Black, the godfather to Harry Potter. Due to how difficult escaping the prison was, most wizards lived in fear of being sent to Azkaban; leading to less crime among the Wizarding World, something that was made abundantly clear through Sirius Black's tragic arc.
What Happened To Azkaban After Voldemort's Defeat?
Voldemort's death signaled an end to the Second Wizarding World and a resurgence of peace. While lots of powerful Harry Potter wizards had died and many families had been irreparably broken, the Dark Lord's death offered a chance to begin anew. One of these ways was in reformation of Azkaban; both the building itself and the rules governing the prison.
Firstly, all the prisoners who had been incarcerated under the Muggle-Born Registration Commission were set free. The Muggle-Born Registration Commission was a law passed during the Second Wizarding World that led to the incarceration of hundreds of Muggle-born wizards simply because of their roots.
This doesn't mean their cells lay empty however, as these prisoners were instead replaced with the surviving Death Eaters and followers of Voldemort's army. Also adding to the inmate pool were Ministry officials who had gone along with Voldemort's will and assisted through passing laws that helped the Dark Lord's cause. Dolores Umbridge, who had been responsible for the Muggle-Born Registration Commission and is seen by many Harry Potter fans as being scarier than Voldemort, was also incarcerated in Azkaban.
Following this, the new Minister of Magic, Kingsley Shacklebolt, passed it into law that Dementors would be driven away from Azkaban, no longer trusted to serve guard due to their faltering allegiance when Voldemort returned. Instead, Azkaban's guards were replaced with Aurors, who would patrol the prison and rotate between staying at Azkaban or the mainland. Under the new status quo, Azkaban suffered no outbreaks over the following two decades.
Azkaban Remained A Lonely, Cold Place
While Azkaban without any Dementors probably seemed like a walk in the park for criminals, especially those who had experience staying there when Dementors were the guards, the prison still proved to be a lonely, cold place. It is unlikely that any prisoners were able to enjoy their serving time, and the prison remained a heavy consequence of crimes, deterring wizards from committing them to begin with.
Though the time period following the Second Wizarding World was less harsh, Azkaban remained an unsavory place that kept British wizards in line.