The nickname of Ivan the Terrible is this trope. He was indeed feared by his subjects, primarily not because of his total death toll – his reign coincided with the golden age of Inquisition and the struggle between Catholics and Protestants in Europe, so he was a rather moderately brutal king by his century’s standards. But he had an unpredictable personality, was prone to onslaughts of rage and liked the more alternative methods of capital punishment. Likewise, in order to crush his Boyar monarchs, he innovated his personal State Sec the Oprichniki who were Torture Technicians and essentially a Cult dedicated to serving the Tsar.
Fun times, if you get one of the less friendly Emperors to rule. Just set the Inquisition after psions, instead of heretics…
Swiss mercenaries were considered the best soldiers in the world for several hundred years. To make sure everyone on the battlefield knew exactly who they were, they took to wearing outrageously colorful uniforms. The uniforms of the Vatican’s Swiss Guard is a relic of this custom.
They were so feared that a treaty ending a war between several major powers specifically stated that no one was allowed to hire Swiss mercenaries. Ever.
Now, your PC mercenaries have a goal to reach for.
Rozalia Zemylachka, one of the last of the “Old Bolsheviks” to escape being purged by Stalin. Why? Because he was terrified of her. She was known as an extremely competent and sadistic soldier, and moreover one who was relatively content with her station in the Soviet state. She was just too psychotic to bother: as Verbal notes in The Usual Suspects, “how can you shoot the devil in the back? What if you miss?” There was a good chance any attempt on Zemylachka’s life would result in the death of the entire arresting force, and an extremely pissed-off and perhaps mildly amused killing machine out for revenge.
This would make a ferocious and memorable enemy for the PCs.
During The Crusades, men in an advanced stage of leprosy were given a sword, some basic armor, and one last chance to serve God and country by being a soldier. Due to the disease destroying the parts of their brain that allowed them to feel pain, while still being able to follow orders and swing said sword, they became terrifying fighters that were part Implacable Man, part walking biological weapon, as every drop of their blood that was spilled held a very high chance of infection. Most just ran for the hills rather than risk being slowly eaten alive by leprosy.
Disease-carrying fanatical soldiers, who happen to be immune to pain. “Fun times.”
Since the 1960s, military attack helicopters have been bringing nightmares to ground forces, especially insurgents lacking the weapon necessary for fighting back. Most attack helicopters are nearly impervious to anything short of a dedicated anti-air missile and have enough firepower to wipe out an entire city block. Several notable examples include the Mi-24 Hind and the AH-64 Apache.
Most grav tanks are designed to fight other grav tanks, but there are exceptions.
To the various terrorist and insurgent organizations that the United States has opposed in the 21st century, the Fairchild Republic A-10 Thunderbolt II “Warthog” is terrifying. It’s one of the last dedicated ground attack jet aircraft and uses a giant explosive chain-gun and a compliment of missiles to devastate nearly any ground target. This isn’t why its particularly feared, however: its massive amounts of titanium armor and durability are. In tests, this aircraft has been able to soak up multiple air to air and ground to air missile strikes and keep flying, and can even do so with large parts of its wings and one of its weapons missing. Even most state-sponsored armies in the Middle East simply lack the firepower to take one of these things down.
And now you know what tech levels are all about.