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The archetypal cavalier excels at mounted combat, but this is only one of the cavalier's abilities. Usually born among the nobility and raised at court, a cavalier is equally at home leading a cavalry charge or exchanging repartee at a state dinner. Cavaliers also learn how to guard those in their charge from harm, often serving as the protectors of their superiors and of the weak. Compelled to right wrongs or earn prestige, many of these fighters leave their lives of comfort to embark on glorious adventure.
Greyhawk: Cavaliers are typically feudal knights fighting by traditional means. Skilled at a powerful charge, but most of all at holding a line of inferior troops or rash adventurers safe and in good order. Among mercenaries and other professional fighters, this is considered old but true, a cavalier is very much a respected warrior.
Source: Xanathar's Guide to Everything
When you choose this archetype at 3rd level, you gain proficiency in one of the following skills of your choice: Animal Handling, History, Insight, Performance, or Persuasion. Alternatively, you learn two languages of your choice.
Born to the Saddle
Starting at 3rd level, your mastery as a rider becomes apparent. You have advantage on saving throws made to avoid falling off your mount. If you fall off your mount and descend no more than 10 feet, you can land on your feet if you’re not incapacitated.
Finally, mounting or dismounting a creature costs you only 5 feet of movement, rather than half your speed.
Starting at 3rd level, you can menace your foes, foiling their attacks and punishing them for harming others. When you hit or miss a creature with a melee weapon attack, you can mark the creature until the end of your next turn. This effect ends early if you are incapacitated or you die, or if someone else marks the creature.
While it is within 5 feet of you, a A creature marked by you has disadvantage on any attack roll that doesn't target you.
In addition, if a creature marked by you deals damage to anyone other than you, you can make a special melee weapon attack against the marked creature as a bonus action on your next turn. You have advantage on the attack roll, and if it hits, the attack's weapon deals extra damage to the target equal to half your fighter level.
Regardless of the number of creatures you mark, you can make this special attack a number of times equal to your Strength modifier (a minimum of once), and you regain all expended uses of it when you finish a long rest.
At 7th level, you learn to fend off strikes directed at you, your mount, or other creatures nearby. If you or a creature you can see within 5 feet of you is hit by an attack, you can roll 1d8 as a reaction if you're wielding a melee weapon or a shield. Roll the die, and add the number rolled to the target's AC against that attack. If the attack still hits, the target has resistance against the attack's damage. When you use Warding Maneuver to protect your mount, you don't need to use your reaction.
You can use this feature a number of times equal to your Constitution modifier (a minimum of once), and you regain all expended uses of it when you finish a long rest.
At 7th level you can take an additional reaction each round that can only be used to make an opportunity attack or to use Warding Maneuver, above.
Hold the Line
At 10th level, you become a master of locking down your enemies. Creatures provoke an opportunity attack from you when they move 5 feet or more while within your reach, and if you hit a creature with an opportunity attack, the target's speed is reduced to 0 until the end of the current turn.
Starting at 15th level, you can run down your foes, whether you're mounted or not. If you move at least 10 feet in a straight line right before attacking a creature and you hit it with the attack, that target must succeed on a Strength saving throw (DC 8 + your proficiency bonus + your Strength modifier) or be knocked prone. You can use this feature only once on each of your turns.
Starting at 18th level, you respond to danger with extraordinary vigilance. In combat, you get a special reaction that you can take once on every creature's turn, except your turn. You can use this special reaction only to make an opportunity attack, and you can't use it on the same turn that you take your normal reaction.
Another subclass that looks bad, my changes involved removing the number of daily uses on Unwavering Mark and Warding Maneuver. Not only does this make sure you have the ability even in the final fight, it also means you don't have to ration the use early on. The end result should be that these abilities see a lot more use. But they are still both circumstantial.