The Conqueror

Throughout history, there have been a few individuals who have had a marked personal effect on the course of events around them. Some did this through political or sometimes aesthetic means, but most of those who were single-handedly instrumental in deciding the path of history have been military leaders. It seems that certain men combine a rare set of natural traits - leadership ability, tactical thinking, staggering ambition, copious ego, and a large dose of suicidal bravery, all combined with a visionary drive to rule the world and the imagination to see it done - to create a whole individual who is patently capable of turning even a ragtag band of woefully underfed and poorly equipped vagabonds into a fighting force that is not only formidable but easily capable of crushing any opposition that stands before it.

A true Conqueror strives to claim the whole of the known world through military force, and often succeeds. (Note that he often doesn't put much thought into running it once he's taken it; he just wants the bragging rights). This is in contrast to those characters who rise to universal power through other means (such as subverting the democratic process or literally being appointed dictator by a legislative body).

The easiest way to recognize this character is by what he manages to accomplish. If he doesn't take over two or three countries (at least) in the course of the story, he's not The Conqueror.

Most Conquerors are by nature Magnificent Bastards. Some even dip into Omnicidal Maniac territory. They are also their own Trope Namers. Quite often, these characters appear in the Backstory of a setting, and all that is left of their conquests is a Vestigial Empire.

Conqueror From the Future is a Sub-Trope of this when the Conqueror in question has, in fact, actually managed to conquer something. Galactic Conqueror and Young Conqueror are specific variants. Multiversal Conqueror is this trope taken to its logical extreme.

Not to be confused with the movie starring John Wayne as Genghis Khan, although Genghis Khan was a Conqueror by this page's definition.

Examples of The Conqueror include:

Animation

Lelouch of Code Geass tried and aimed for this. He didn't particularly need to be in control, of course.
Reign: The Conqueror has Alexander the Great as the main character.

Comic Books

Marvel Comics has Kang the Conqueror, Rama Tut, the Scarlet Centurion and Immortus. The catch is that these are all actually the same guy: He's traveled through time so often, and created so many Alternate Timelines, that there is now an entire Legion of Doom called the Council of Kangs made up entirely of his own iterations. Immortus, it seems, is the original Kang, who is now a Boxed Crook: forced to spend eternity undoing the Continuity Snarl that is the Marvel universe thanks largely to him.
In the DC One Million story arc, is it revealed that Vandal Savage, an immortal, evil Julius Beethoven Da Vinci who has been alive since 50,000 B.C., has finally managed to Take Over the World. And he hates it.

Literature

General Tacticus, from whose name the the word "tactics" is derived, was a famous Morporkian leader of the Discworld. Not only did he win countless wars for the Empire of the time, but when placed in command of a satellite city-state, he promptly did what he regarded as the best move in the interest of said state's defensive self-interest, and attacked Ankh-Morpork. And won, of course. He also famously wrote a book that is the Disc's equivalent of The Art of War, which is standard reference material for military leaders the Disc over.
Artur "Hawkwing" Paendrag of The Wheel of Time not only managed to take over the entire continent, but also ran it so well that "a child could carry a bag of gold from one coast to the other without fear", although his justice system became a bit draconian in later years. The only group he failed to conquer were the Aes Sedai, and possibly only because he died before his war against them was concluded. Unfortunately, he succumbed to the Vetinari Paradox, and his hard-won Empire collapsed soon after he died.
The Malloreon had Emperor Korzeth, who managed to unite his continent after their patron god and king, Torak, was felled (but not killed) in battle, spending his entire life in a series of bloody wars so that he could pass the empire on to his descendants. His ultimate descendant, Zakath, had aims to finish the job and take the whole planet, but his ambitions cooled over time.
Part of the irony in Percy Bysshe Shelley's Ozymandias is that the empire of the Great Conqueror Ozymandias is now nothing more than sand and ruins. The message being that all things pass away, even Empires.
Aegon the First, also called The Conqueror, in A Song of Ice and Fire.
Kieron in Steven Brust's Dragaera. In addition, Sethra the Younger is a wannabe The Conqueror, though she has not succeeded (yet).
Robert Aspirin's Myth Conceptions introduces Big Julie, the general in charge of one of the largest armies anyone has ever seen. He's conquered a fair bit of the entire world by the time he's stopped by the heroes. Of course, he's a Captain Ersatz of Julius Caesar.
In the Backstory to the first Gor book, Marlenus, the Ubar of Ar, has captured the Home Stones of about a hundred other city-states, which in practical terms means those city-states are vassal to Ar. It is Tarl's assignment to steal Ar's Home Stone, thereby breaking their power.
In the distant backstory of Deep Secret, there's Koryfos, a pseudo-Alexander the Great who conquered entire universes. By the time of the story, his empire has dwindled and is on the verge of total collapse. Then he comes back at the end to fix it.
In the Black Company series, The Lady qualifies as this, having subjugated an entire continent. In the past, her husband The Dominator was even worse.

Live-Action TV

Xena: Warrior Princess featured an Alternate History storyline in which Hercules was never born, and thus never "unchained" Xena's heart. She ended up the cold and ruthless Conqueror. As the name implied, she brought most of Ancient Greece and the Mediterranean region under her boot.
- Likewise, in "Hercules: The Legendary Journeys, the Parallel World version of Hercules, the Sovereign, is a ruthless conqueror.

Tabletop Games

In the official setting for Champions, there's Istvatha V'han, "The Empress of a Billion Universes", an immortal time-travelling Dimension Lord who really has conquered a billion other dimensions.
One of the example/scenarios in GURPS Shapeshifters is a parallel Earth where a shapeshifting alien masquerading as a human has essentially taken over the world; he's even nicknamed "The Conqueror".

Video Games

Lord Median the Conqueror from Soul Nomad and The World Eaters united the game's war torn world under his own rule.

Real Life

Napoleon Bonaparte, who almost succeeded in uniting Europe under his rule, if it weren't for those pesky British and a bad winter in Russia.
Julius Caesar. So effective his name became a title for emperors after him.
Alexander the Great, who holds the record for most land personally conquered by a single individual.
Genghis Khan, who ruled an empire that made Alexander's look like a postage stamp (but who did not personally conquer all of said land).
- What's said above under Literature about Artur Hawkwing's realm is actually adapted from a boast Genghis Khan made: "A virgin with a bag of gold around her neck could walk naked from one end of my realm to another without being attacked."
King William I of England (better known as William the Conqueror), first battled to secure his claim on Normandy, then invaded England in 1066. While he really only conquered one small island, the impact of that one small conquest on world history cannot possibly be overstated.
- Prior to being known as William the Conqueror, he was known as William the Bastard. No doubt he really appreciated the switch in titles.
Charlemagne, King of the Franks and later the original Holy Roman Emperor.
During his reign (1479 - 1425 BC), Pharaoh Thutmose III conquered Northern Africa and the Middle East from Niy in north Syria to the fourth waterfall of the Nile in Nubia, the largest extent ever covered by the Egyptian empire.
Cyrus the Great created the Persian Empire by conquering pretty much every other civilization in Southwest Asia, the Near East, and the Middle East, from Egypt to the Indus River. Furthermore, he was a savvy statesman able to give his empire a lasting stability by being generous to the conquered peoples enough to secure their loyalty.
- Darius, Cyrus's successor, added parts of Greece and even more of India to Cyrus's empire.
Attila the Hun carved out an empire that stretched from Germany to the Ural River and from the Danube to the Baltic Sea.
Isabella of Castile and Ferdinand of Aragon united Spain under their joint banners and drove the Muslims out of Western Europe. And then they subsidized an ocean voyage by some guy named Columbus. (Modern Spaniards like to give the credit to Ferdinand alone because Isabella was an icon of the Franco regime, but Isabella was a far more capable ruler and a better general to boot; while she was leading troops (often while pregnant), he was off porking his various mistresses before showing up after the battle to take credit.)
Suleiman the Magnificent expanded the Ottoman Empire by basically eviscerating the Byzantines. Under his rule, the Ottoman's controlled everything from Algeria to Iran and from Ethiopia to Hungary.
- In a bit of a departure from some of the other Conquerors here, Suleiman was also known for the great improvements he made to the administrative system of his country. While in the West he was Suleiman the Magnificent, in the East he was known as Suleiman the Lawgiver.
Adolf Hitler accomplished what Napoleon Bonaparte tried to do: he ruled a Europe united under one banner. Surprisingly, he was overthrown by the same factors (that is, not being able to conquer the British and, most importantly, violating Montgomery's Second Law of War: "Never march on Moscow."