While cults aren't particularly common in anime, their inclusion makes for some seriously twisted storylines. They can produce some of the creepiest villains imaginable, even though some of them turn good when the brainwashing wears off. They can also provide some truly sympathetic backstories for all kinds of characters.
While some use the word to refer to any new religion, a cult needs to have a few specific features in order to qualify. According to psychiatrist Robert Jay Lifton, a true cult requires a charismatic leader, an initiation ritual that brainwashes inductees, and coercive motives. Anime cults manage to achieve all of these prerequisites, and because they exist in a fantastical, animated world, their sick goals often put the most terrifying real-life cult stories to shame.
SCRATCH is a cult that appears in a single episode of Cowboy Bebop, and it has an unusual goal. Members of SCRATCH want to digitize their consciousness, which elliminates the need to maintain their human bodies. Faye pretends to join, claiming that she wants to get rid of her body so she can clear all her debts, but really she's after Dr. Londes — the cult's leader — who has a huge bounty on his head.
As it turns out, Dr. Londes is actually a child in a vegetative state, whose true goal is to separate people from their bodies the way that he has been separated from his own.
While it's not the primary focus of the show, Death Note does feature the Cult of Kira, a group that worships Light Yagami's killer alter ego, Kira. Many Kira worshippers are simply grateful for their God, who gets rid of the people who make the world a dangerous place. Others — like television personality Hitoshi Demegawa — use their devotion to Kira to gain fame and recognition for themselves. In the case of Hitoshi, his prestige makes him a target for Kira, who ultimately kills him.
Fairy Tail features the Avatar Cult, which worships the dark arts and their creator, Zeref Dragneel. They also want to conquer the world in Zeref's name. Specifically, they want to purge the world of infidels (those who don't worship Zeref) and attract the subject of their devotion to them via bloodshed.
The Witch Cult of Re:Zero worships the Witch of Envy, Satella, who they aim to resurrect. The cult has existed for about 400 years, and its top members are known as Sin Archbishops. Each of these archibishops represents one of the seven deadly sins, which the ancient witches used to embody. The members of the cult are reclusive, and tend to avoid the outside world.