Cthulhu Invictus is a horror fiction role-playing game based on H. P. Lovecraft's story of the same name and the associated Cthulhu Mythos. The game is published by Chaosium with supplemental sourcebooks by Golden Goblin Press. The game was released in 2009 and most of the additional sourcebooks and campaign/adventure books since then have been published by Golden Goblin Press.
The game takes place in the world of the Cthulhu Mythos, though in the time of the Roman Empire. It mixes the Cthulhu Mythos with the mythology of the ancient Greeks, Romans, Celts, Germans, Egyptians, and so on. With such classical monsters and creatures like the Satyr and Harpy included in the setting along with the monsters and entities of the Cthulhu Mythos. Scenarios run the gamut from conspiracies in the Imperial capital (Rome) to sinister cults in the outskirts of the empire.
Cults that worship the Great Old Ones or Outer Gods may be outlawed and therefore operating underground, or officially tolerated or supported by the Imperial authorities. Barbarian tribes often worship one of the Mythos entities and the cults operate in foreign kingdoms. Orgies and human sacrifice in their name happens both within the cities and on the frontier. Deep Ones trade and interbreed with some of the coastal communities. Outside of the cities, it's the Roman soldiers that investigate and hunt down the cults and do battle with the monsters when encountered or their barbarian worshipers (if they don't get slaughtered first).
Fishmen of Lierganes = It's a fishing village in Hispania on the Mediterranean, whose inhabitants hundreds of year ago made a pact with the Deep Ones. In return for worshipping Dagon and allying with the Deep Ones, they would receive more fish each day than they catch per month. As the years passed, their children began to change. Deep one hybrids eventually outnumbered the untainted. Now they capture women to use as breeding stock and raid Roman trading vessels.
Daughters of Isolation = Which is an all-female cult in Antioch (Greece) whom follow the Great Old One Ut'ulls-Hr'ehr who lives in the caverns below. Twice a month they wear masks which are made of flayed human skin and wear sheer gowns. The women lure men to the Paradise of Daphne, promising that they will fulfill their most outrageous sexual desires. The orgy is a cover, and three victims are taken underground. Whom have been drugged, becoming semi-conscious and unable to either speak or move. They are both raped and flayed before Ut'ulls-Hr'ehr and then fed to it.
Followers of Dagon = Consists of a hundred humans and deep one hybrids living in Tyre (Phoenicia) who worship Dagon. Underwater caverns and tunnels connect the Mediterranean sea to the temple, which the Deep Ones use to travel to the temple for mating and worship. The Deep One city raids grain shipments from Egypt. From the ships they take slaves and breeding stock, and give salable items (e.g. grain) to the temple which can be sold to human traders.
Followers of Urakhu = Sorcerers who worship the Great Old One Shudde M'ell. It's a cult which has existed for thousands of years. Which was founded by desert nomadic tribes, whom built a great city in the southern Arabian Peninsula, the city of Iram. Every five days the sorcerers will sacrifice to Shudde M'ell a non-tribe member.
Republicans = It's a conspiracy of Senators and former Senators, with the goal of overthrowing the emperor and reestablishing the Republic. They control several legates and thereby the Legions under their control. Despite their formidable resoures and deep financial resources, they are far from achieving their objective. Some of its members are impatient and desperate, and have been in contact with Samaritan and Etruscan magi. They hope to use ancient powers to hasten the fall of the Empire, without understanding the dangerous forces the magi are attempting to harness. It's active in Italy, Hispania, Judea, Egypt, Greece, Northern Africa, and Asia Minor.
Priests of Melkarth = Which is a cult in Tyre (Phoenicia) that consists mostly of wealthy, powerful citizens whose temples are found across Syria. Though it a cult that publicly worships Melkarth, in reality it worships Cthugha, a fact known only to its priests. Whose ultimate goal is to summon Cthugha, to overthrow the Romans, and take over the world via its powers. In public rituals, animals and grain is sacrificed to Melkarth. Privately, they sacrifice kidnapped children. They have successfully summoned the Children of Cthugha to guard the temple. It's active in both Syria and Northern Africa.