In the Majestic Wilderlands supplement I detail a knight class. The basic concept is a fighter that is good at mounted combat. For a thousand years from the Battle of Adrianople to the last Middle Ages the mounted knight was the premier weapon on the battlefield.
I draw a distinction between the knight as a weapon and a knight as a social institution. The knight class that I created is focused on the Knight as a weapon. Whatever social conventions that surrounds this class will need to be created for your campaign.
But the implication of being a knighted warrior is that your status is fairly high. A horse require care and food, as much as a person per year. Also you just can't have one horse as a matter of practicality you will need multiple mounts. They are not cars to be ridden at need and then put away. In most societies a mounted warrior has some measure of status and wealth. With status come a bonus in charisma. This is reflected in the Charisma requirements for a knight. A fighting-man needs a constitution or charisma of 12 or better to start out as a knight. And neither can be lower than 8.
Over the centuries to the knight status was attached a lot of mysticism and religion particularly during the High Middle Age and the rise of the ideal of chivalry and the romance.
In the Majestic Wilderlands the fundamental essence of a knight is that he is his liege's man. To fight at his liege command in the place and time of his liege choosing. It is the reason he given status, power, and equipment. In short he there to kick the shit out of those that piss his liege off. Various cultures may obscure this with social conventions but this is the heart of knighthood.
The Tharians were horse barbarians. In their society status directly correlates to the size of your herd. If you have no horse you are not a warrior and without honor. Tharians that have the misfortune to lose their last horse often join the Beggars, wandering groups of clanless, honorless Tharians.
Tharians organize themselves into clans. A clan is basically several extended families sharing a common ancestor. Existing alongside the clan are the housecarls. These are warriors who have sworn personal loyalty to the clan chief. However this loyalty isn't a one-way street. The housecarls expect their chief to be generous and reward them greatly for their service. The chief who is frugal and stingy is soon bereft of any housecarls. Those that give freely are known for their generosity command the largest contingents.
Since the conquest of City-State this ideal has been under a lot of pressure. For the first time Tharian Culture have a mean of gaining power without having a lot of horses thanks to the money economy they inherited from the conquest. Warrior can be gained by paying them gold and silver. This most pronounced near the heavily urbanized regions near City-State.
This has caused some Tharians to turn away from their traditional religion (ancestor worship) to worshiping Set, the Dragon God. The harsh discipline and stern codes of Set appeals to many Tharians.
Ghinorians are a reflection of the traditional western european d&d fantasy. Instead of a Catholic Church, they have a Church of Mitra. Believing themselves to be the chosen people of Mitra.
The Ghinorian Knighthood had it's origins in the foundation of the half-legendary Kingdom of Ghinor. When they arrived in the Valley of Ghinor for nearly a thousand years they lived in separate tribes ruled by Princes. They fought each other as much as the surrounding barbarians. From time to time there were threats that require the tribes to unite. But because they rarely cooperate the armies lack a lot of cohesion and lost as much as the won.
The Tribe of Loris, what the Church of Mitra was known back then, anointed seven warriors. These seven trained seven more. This group of 49 warriors were known as the Paladins. The paladins were the first military force that the Ghinorians had that would fight for all the tribes. To join the paladin was to leave your tribe behind and fight for Ghinor and Mitra.
Their force grew and became highly effective when they adopted chariots. Their mobility allow them to respond to threat quickly regards of how far away a tribe was. The Paladins became a source of unity and from their leadeship came the first Kings of Ghinor.
Eventually the Kingdom of Ghinor passed away but the ideal of the Paladins lived on. During the rise of the Ghinorian Empire, the Imperial Prince split the Paladins into two groups. The first retained the name of Paladin and focused on the protection of the Church of Mitra. The second group became Knights in service of the Imperial Prince fighting the wars of the empire.
This division continued after the Empire's fall. The paladin grew into being holy warriors fighting Mitra foes in the Wilderlands. The Knights focused on the defense of the successor realms and upholding Mitra's justice among the Ghinorian people.
In Elessarian lands Knights function purely as police and soliders. Unlike Tharian and Ghinorian lands, the judgment of the law in the hands of the Trehaen a druidic order. They consider the Knight an office than an all-encompassing mark of status.
Among Sylvan realms, lands dominated culturally by the Elves, a Knight is a mark of honor. Granted to warriors that showed exceptional valor or service. Rangers are warriors in service to the god Silvanus, the patron of elves. They are noted for aiding any allied realm regardless of race.
In Viridistan a Knight is a military office denoting a mounted warrior. It carries some status because of the requirements of keeping up a horse but no more than being a officer in a modern military would. This is similar to how the equites of the Roman Republic worked.
Dwarves, Skandian, and other barbarian societies there is little concept of knighthood. Instead prestige is earned by reputation alone. Like the Tharians, chiefs known for their prowess and generosity will attract a warband loyal to him.
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