The Dementors are some of the most frightening creatures encountered in the Harry Potter universe, a collection of seemingly cloaked ghouls who literally feed on happiness itself. With those abilities, it makes perfect sense why they're feared. But a fan theory might have found a way to make them even scarier by suggesting how they can turn their victims into Dementors like themselves.
Within the Harry Potter series, the Dementors are some of the most fearsome creatures encountered by the Wizarding World. Introduced in Harry Potter and the Prisoner of Azkaban as the guards of Azkaban prison, the wraith-like creatures feed on the joy of human beings, leaving them drained and incapable of resistance. Dementors are even capable of performing the dreaded Dementor's Kiss, which sees them fully drain the victim's soul from their body -- leaving them a husk of the person they once were. Although Harry (and by extension, the audience) learns about the creatures and specifically how to combat them, their origins are left shrouded in mystery within the series.
As broken down by J.K. Rowling in a story for Pottermore, Azkban's origins were revealed to have been the result of an unstable dark wizard named Ekrizdis, who'd lured Muggle sailors to his island in the North Sea centuries before Voldemort or Grindelwald became threats. When the island was discovered, the ruins were filled with Dementors, already known to the Wizarding World and rightfully feared. Concerned destroying their apparent habitat would result in Dementors venturing toward the mainland, they were left alone and eventually made the guards of the newly constructed prison made atop Ekrizdis' island. Their true origins were not revealed in this story -- but according to a theory from Reddit user themorallessdoctor, the early Dementors found on the island could have been increasing their numbers all along.
The theory focuses on the specifics of the Dementor's Kiss. As themorallessdoctor explains, the Dementor's kiss leaves someone a shell of themselves but still notably alive. This is the fate that befell Barty Crouch Jr. in the novel version of Harry Potter and the Goblet of Fire. It leaves the victim more or less "empty," which could mean they are capable of being filled with dark magic. This process warps the person's remains into a new Dementor -- continuing the cycle.
In effect, Dementors can turn people into Dementors by stealing their souls. The theory even notes that Ekrizdis' overwhelming dark magic and sadistic actions in his era might have resulted in the creation of the early Dementors, the remnants of his victims twisted by his magics. The books mention Dementors being spawned like fungi in places of decay, but it's an interesting wrinkle that some Dementors can actually be created rather than born.
It adds a new layer to why witches and wizards would be so frightened by the concept of encountering Dementors because they can transform not just their surroundings but the people around them. It also details what happens to those afflicted by the kiss and completely justifies the idea that such a fate is far worse than death. The theory clarifies exactly why wizards and witches would accept somewhere like Azkaban as a reality if it meant containing the threat to a solitary location. It also explains why the Dementors can look so human-like in general build -- as some of them may have even been once human. The possible origins of Dementors are still mysterious, but this theory finds a way to explain how they can spread so easily while also making them far more terrifying to consider.