The Titan Cult, also known as the "Titan Worshippers", are a suicidal and violent cult of people within the walls who view the Titans as gods and an antagonistic faction in the Attack on Titan universe.
During the events of Before the Fall, the Cult's new leader, Elena Munsell, opened the gates of the Shinganshina District to allow Mammon access to their city. Though they were able to murder the governor of the city, most of the cult's members were wiped out by the Survey Corps, though it was too late as Mammon was already devouring the district whole.
When Dario was buying Kuklo from an old man, the man stated that someone else had tried to buy Kuklo before, and he was "a gloomy man in black rags". This is implied to be a member of the Titan Cult.
Then, 15 years later, more Worshippers attack the Inocencio household, and kill many servants as well as Dario Inocencio. Many of their members were killed in the attack, likely by Xavi, as he defended his sister, Sharle, and himself. According to one of the Worshippers, they were after the "Titan's Son".