Luckily John Ostrander gave us an interesting new option during his run on the Suicide Squad (which I have talked about before) by introducing the Thuggee assassin Ravan in the first arc of that book. He later went on to become a member of the team and added an interesting moral dimension to the team.
There may be historical issues with the existence of the Thuggee, but that rarely gets in the way of good gaming (or comics for that matter). What I liked about Ostrander's incorporation of the cult into the DC Universe was the idea that, while they ostensibly worshiped the dread goddess Kali, their worship was out of fear and the murders that they committed were sacrifices to her in order to forestall the end of the current age and the start of the Age of Chaos, the Kali-Yuga. Ravan's "catchphrase" was "A Thousand Years, O Kali," because each consecrated death to their goddess would stall the coming of the Kali-Yuga by another thousand years.
Unlike the ninja, there is no running around in their underwear and using of ancient, outdated weapons for this cult. Their mixture of the ancient and the modern makes them an interesting foil for high tech super-heroes, or espionage agents.
So, how do you do this in your campaign? You could easily just reskin whatever passes for a ninja in the existing writeups for the system you are playing and add the bits about "killing for a higher cause" to them. Bam. You have Thuggee. When I introduced them into my Marvel Super-Heroes campaign back in college, that was basically all that I did. I think that my writeup for Ravan was cribbed from the one for The Taskmaster, removing his powers. A Ravan-like enemy should be capable of going toe-to-toe with a Daredevil or Bronze Tiger, but should be outclassed by a Batman or Captain America. Throw in a half dozen generic Thuggee to round things out for the player characters.
When will you use these Thuggee in a campaign? Their "calling" makes them a little more complex than your typical smash and grab type of villain. You can even make the Thuggees in your campaign world into a sort of morally grey hero, after all they are trying to keep the destruction of the world at bay and stop millions from being killed when a mad death goddess incarnates in the world. Sure, their tool for doing this is to kill people, but sometimes you have to break a few eggs to make omelettes. These killers consider themselves to be holy warriors on a mission, and the super-heroes are at odds with that mission.
This can add an interesting moral dimension to a super-hero campaign, particularly if the Thuggees only target evil or criminal individuals for their sacred killings. If there was demonstrated proof that gods like Kali really existed in the world, would that make a hero's choice to stop a killer from killing a killer a more difficult one? It would certainly make it a more interesting choice. Super-heroes, particularly in role-playing games, tend to have a black and whiteness to their morality. The simple addition of a faction like this can spice a game up considerably.
This can make a character who is like the Paul Kirk version of Manhunter from the 1970s revamp of the character. The hero is fighting a silent, shadowy war against the cult, which occasionally erupts into the streets of the city, or on some espionage mission, drawing the heroes into the action and giving the former Thuggee some allies for a time.
And if you would prefer to not integrate a real world religion or goddess into your games, then substitute a fiction god or goddess for Kali. Imagine a version of the Cthulhu Cult that isn't trying to raise their god from his watery grave in R'lyeh, but is instead trying to keep him from rising and destroying their world. Imagine a cult that sprung up out of the end of the novel Dracula (or any of the countless adaptations if you prefer) that is taking the blood of victims so that the Count will not once again rise and make his vampiric armies. A concept like this has a lot of applications to a number of different genres of gaming. The idea is that the deaths caused by this religion is supposed to serve a greater good, and by interfering with them the heroes may be dooming their own world.
Of course, it could all just be a lie and, no matter what the members of the religion believe, there is no actual god or goddess or future destruction that they are forestalling, and their murders don't make them any better than any other killer.
Regardless of how you use this concept in your games it will add an interesting morality to them. My only real recommendation would be that whatever variant of this cult that you use in your games, you make them NPCs and any PCs are former members of the group.
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