Have you noticed how many stories, whether it be on screen or on the page, are bedecked with secret orders. I am not talking about instructions but groups, clandestine bodies that make up some kind of threat to the status quo in any given situation or time zone.
I remember one of my uncles, I had three and two aunts, was a member of the Freemasons, and that was all hush hush and still is as far as I'm aware. The series Timeless has the Ritenhaus group. James Bond has Spectre; and so on. In fact even my Steele novels has the Gurentai.
In short, a gurentai is a gang in a much more traditional sense, a group of young unruly thugs who peddle their violence for profit. They often engaged in the suppression of unions and other workers' organizations and such activities brought them much closer to the conservative elements of the Japanese power structure.
Being a writer the description didn't fit what I wanted so I changed it! I still like the name though!
Then there is the Knights Templar over which much has been written.
The order, which was among the wealthiest and most powerful, became a favoured charity throughout Christendom and grew rapidly in membership and power. They were prominent in Christian finance. Templar knights, in their distinctive white mantles with a red cross, were among the most skilled fighting units of the Crusades. Non-combatant members of the order managed a large economic infrastructure throughout Christendom, developing innovative financial techniques that were an early form of banking, and building fortifications across Europe and the Holy Land.
The Templars were closely tied to the Crusades; when the Holy Land was lost, support for the order faded. Rumours about the Templars' secret initiation ceremony created distrust, and King Philip IV of France – deeply in debt to the order – took advantage of the situation to gain control over them. In 1307, he had many of the order's members in France arrested, tortured into giving false confessions, and burned at the stake. Pope Clement V disbanded the order in 1312 under pressure from King Philip.
The point is that such undercover groups may add a frisson of the unknown to your writing. The idea that there is a shadow lurking in the background but exercising a modicum of dubious control, is adding tension and excitement.