Subjugatin' ain't easy. You can't just show up and expect everyone to give you their stuff and obey what you say. People ask inconvenient questions like, "who the heck are you?" and "from whence do you derive your authority?" If your reply, "The Lady of the Lake, her arm clad in the purest shimmering samite, held aloft Excalibur from the bosom of the water signifying by divine providence that I, Arthur, was to carry Excalibur. That is why I am your king" doesn't work, it is time to resort to violence.
History is full of brutal subjugation techniques. The tools of psychological terror range from heinous torture to nauseating violence and dismembered corpses. What really seems to do the trick, however, are public spaces festooned with human remains. Nothing enables tyrannical oppression like the public display of dead bodies. When peasants see a dead guy with his eyes gouged out hanging from the castle walls, they're generally too busy pissing themselves to think about rebellion.
Nothing Says "Obey Me" Like a Head on a Spike
There is a long tradition in many societies of putting heads on spikes as acts of intimidation. It just feels natural, implying, "look at this guy. You don't want to end up like him."
The English were probably best at the practice, and took it to a whole new level. London Bridge showcased the severed heads of traitors and enemies of the state for centuries. At one point, there were 30 heads above the Stone Gateway, displayed simultaneously. Some notable heads include those of William Wallace, Thomas More, and Bishop John Fisher.
Of particular interest is how Oliver Cromwell's head (which has its own Wikipedia page, just fyi) ended up on the bridge. Upon his death in 1658, Cromwell, who was once head of state of England, Ireland, and Scotland, was buried like royalty at Westminster Abbey. A year later, though, his son was overthrown, and King Charles II returned from exile to reestablish the monarchy.
Suddenly, many of the people who Cromwell really pissed off were back in power. They took the opportunity to exhume his corpse, cut off his head, and stick it on a spike over London Bridge. This must have pleased more than a few Irish ("Cromwellian conquest of Ireland" is also a Wikipedia page). The message was sent: Don't mess with the monarchy.
King Edward Longshanks Had "Traitors" Hanged, Drawn, and Quartered
King Edward I (yes, the guy from Braveheart) invented this method of execution because he didn't want anyone to get in the way of his conquest of Great Britain. He first used it on the formerly independent Prince of Wales, Dafydd ap Gruffydd, and later on William Wallace (Mel Gibson from Braveheart). Because Longshanks declared treason a triple crime (against God, man, and the king), it required a triple execution.
First, the condemned was chained, prostrate, to a glorified fence post, and drawn through the streets by horses, so loving townsfolk could stone and mock him. When the The Man had enough of that, the victim was hung by a rope, and his sensitive parts were removed, i.e. emasculation (if you haven't figured it out, they cut his d*ck and nuts off).
That humiliation complete, the condemned was cut down for a primitive version of vivisection; his stomach was cut open and his entrails removed and burnt. Then, the executioner cut open his chest and removed his (ideally) still-beating heart, holding it up to the crowd. Of course, that wasn't enough, so the condemned was then decapitated and quartered (cut into pieces).
Oh, and this was still practiced as an official execution method in England as late as the 18th century. Suddenly, the 8th Amendment's prohibition of cruel and unusual punishment makes a lot of sense. After all, the guys who wrote it were considered traitors to the crown.
The Romans Crucified Just About Anyone Who Pissed Them Off
Jesus Christ wasn't the only guy to get the business end of the crucifixion stick. Crucifixion was a common, horrifically brutal, subjugation technique of the Romans. The process was severe; the condemned was beaten with varying degrees of severity, then forced to carry the heavy cross or post to the execution spot.
Upon arriving at the execution site, the condemned was strung up at one of numerous angles, including upside down, sideways, and Jesus style. Nails were driven through the palms at a nerve, to cause the most pain. This induced involutnariy palm clenching. Some evidence suggests feet were nailed to the sides, not the front, of the cross, through the ankles. Then the condemned was left to die a slow, agonizing death as the lungs filled with fluid in full view of the public.
Today, ISIS is apparently taking a page from this book and crucifying people. Though the Syrian Observatory for Human Rights believes the victims are executed before crucified, the message is still pretty straightforward: "Obey."
Vlad the Impaler Really Earned His Nickname
Good old Vlad III of 15th century Romania was brutal enough to inspire the character Dracula. In fact, the name Dracula comes from Vlad's father's position in the crusading "Order of the Dragon" (Dracul). Dracula means "Son of Dracul." Vlad III spent some time fighting the Muslim Ottoman Empire and some time fighting Christian forces as an ally to the Ottomans, and in both cases, he was crazy about murdering people.
Vlad consolidated his rule and struck fear into enemies and subjects by impaling people. Fields of them, in fact. Rows of stakes were planted vertically, and victims lowered onto them. Usually, the spike entered the rear and exited the mouth (ass to mouth?), but many other angles of impalement were also used. Stakes were oiled and not too sharp, so as to to prevent sudden death from shock. Once impaled, victims were left to die a slow and agonizing death, after which they rotted among their fellow impaled corpses.
Also worth pointing out - in the image above, Vlad is straight up just having a meal at a nicely set table next to his field of impaled victims, while an underling chops up corpses right in front of him. It's probably not historically accurate, but it raises more questions than it answers.