Note: This page contains spoilers for the plot and one of the central mysteries of the story. Proceed at your own risk.
A group of people, in contemporary times mostly British aristocrats, working for the survival and flourishment of humanity. They want self-determination for humanity and seek to bring down Meritocratic rule.
The Cult of Hades worked with black cowled figures from other planes, called "squizzards." The squizzards were only allies of the Cult of Hades, but when most of the actual members fell to the blue veins, Barret and the squizzards became the main representatives of the cult, despite not technically being in it.
The Cult of Hades' history is shrouded in obscurity. The Cult of Pluto, in Ancient Rome, primarily worshiped Persephone. They were made up of entirely clerics rather than clerics and paladins. They were responsible for burial rites, as well as resurrection.
The history the party originally thought to be true was that the Cult of Hades came to Caesar and offered him the chance to defeat all his enemies, and thus brought the monsters upon Rome, pushing Rome into true evil. People believe that there hasn’t been a Cult of Hades since then — the Meritocrats abolished them. According to Barret, the Cult of Hades were magical protectors of Rome and have continued with similar aims to what they had back then. From the Ancient Rome sidequest, however, both of those histories seem untrue, though that's not necessarily proven.
Barret has been working with the Cult of Hades ever since an older crime lord bequeathed him the aid of the squizzard about 30 years ago. They originally “proposed the disappearing” of the children in Other London, including Brock. After the simulacrum appeared, their advice became more insistent, and they suggested that Barret contact the London Rangers in the first place, with regards to the Serpentines. Barret admits that many of his ideas might have not actually been his own while the Cult of Hades was advising him.
The Cult of Hades funded Tesla's development of the Simulacrum and encouraged him to create it. After Tesla realized the scale and danger of the project, the cult passed it over to Edison to finish. Their goal was to use it to topple the meritocrats and gain world domination.
The Cult of Hades sought the Heart of Aphrodite in Cairo, for unknown reasons. During this, they managed to obtain something from the LOLOMG (such as hair or blood) to use as a scrying focus, and began spying on the party.
The LOLOMG discovered a factory belonging to the Cult of Hades in Damascus. They were mass manufacturing Simulacrum. (They also had manticores in warehouses and had laid off the entire previous goblin staff and replaced them with a human skeleton crew.) The LOLOMG began to destroy the factory, but the Cult of Hades attempted to stop them. They kidnapped Barret and offered him to the LOLOMG. When the party still wouldn't cooperate with them, the Cult kidnapped the party's loved ones and held them in Rome, where they attempted to create a Gate spell.
Eighteen months later, the LOLOMG were forced to work with the Cult of Hades (at that point only Barret and squizzards) to save the world.