World conquest in a fantasy setting has often ridden on the backs of horses. Call them what you will: soldiers, raiders, knights, cavalrymen. Real world historical precedent (where fantasy authors get all of our best source material) practically gallops across the map.
Many early empires were born on horseback. The Scythians conquered the Fertile Crescent with their superior horsemanship. The great Mongol khans spread across Asia and into Europe on horseback. The Persians introduced armored cavalry as their deghans conquered and held large swathes of Asia Minor. The Byzantine heavy cavalry (a poor carbon copy of the deghans) held sway over the eastern Roman Empire, then turned and conquered the western half.
The more typical timeline pillaged and re-purposed for classic fantasy is medieval Europe. The image of the knight on horseback is an iconic symbol of medieval cultural history and warfare, though European imperial conquest died out with Barbarossa and would not rise again until Napoleon . . . who made great use of cavalry until the Battle of Waterloo. Horses, horses everywhere.
Let’s take our hypothetical fantasy cavalry. Where do we place them in this rough timeline? Let’s give them a setting rich in hematite and the early ability to produce iron (Hittites) to get a leg up on their neighbors. They’ve developed coordinated tactics, but not large armies (early Roman Republic.) Let’s call them the Knights of the Iron Hills. But ‘knights’ is a bit of a misnomer: they’re raiders with delusions of grandeur.
What spurs these horsemen to reach towards empire? They have the means, but what is their motivation? The Mongols needed land to support their nomadic herding lifestyle. That won’t work for a civilization tied to their cities. The Persians and the Romans had a fondness for spreading their brand of stability and order over a chaotic patchwork of small fractured countries and city states. Destabilized neighbors. Who could resist that sweet, imperial plum when it falls into your lap?
But what is the nature of these fractious neighbors? Let’s start everyone off on equal footing: five kingdoms. Why five? Because three is too few, six is too many, and the five man band played on the continental scale of country level politicking is awesome. Really, they need to be conquered for their own good.
Let’s add some internal strife to the mix. Say the current king of the Iron Hills is weak or passive or just doesn’t want to go to war. All we need now is a young charismatic knight on horseback, a Duke perhaps, long hair whipping in the wind, coming to change things with the thunderous hooves and clashing swords. And from the whole bloody, violent mess, we birth a new empire. But never forget, this empire was birthed on the back of a charging horse.