Titan Cult

The Titan Cult is a violent group of somewhat suicidal people who worship Titans.

Information
It is unknown if the Titan Cult knows much about the Titans or if they see them as some kind of release. They are shown to be quite violent as they killed nearly everyone in the Inocencio household. They also seem to be numerous and somewhat manipulative, as they were able to kill a governor and outmatch both the Survey Corps and the Garrison.

The Titan's Son arc
The worshippers and their new leader, Elena, were able to kill the local governor and open the gates. When they opened the gates, it was likely that most of the worshippers probably died in the initial fight with the Survey Corps and later when the Titan invaded.

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, fifteen 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."