The Master of the Longsword is a warrior who focuses their training on that most foundational of weapons: The longsword in two hands. Training extensively to master the weapon to the fullest of its capabilites, the Master of the Longsword is a consummate duelist who relies on speed, agility, and precision rather than power. Thus, what they lack in their ability to engage multiple threats they makes up for in their ability to overwhelm and dominate a single target.
 
However, because their focus and energy is on mastery of their weapon to its fullest extent, the Master of the Longsword is unable to utilize these techniques with any other weapon.

Restriction

The Master of the Longsword must be wielding a longsword in two hands in order to make use of these techniques.

Scholar of Arms

Beginning at 3rd Level, the Master of the Longsword gains gains three Superiority Dice (1d8) and two Master Strikes, which can be used both offensively and defensively. At Level 10 you gain two additional strike, and at Level 18 you gain a fifth.

Offensively, when you make a melee attack against a target you have Initiative against, you may expend a Superiority Dice to make a special attack from the Master Strikes list. Save DC = 8 + your Proficiency Bonus + your Dexterity modifier. 

Defensively, if the last target you attacked with a melee weapon makes a successful melee attack against you, and there is no other enemy within melee range, you may use a Reaction to execute one of the Master Strike maneuvers to counter your opponent's attack. Roll as if you were making a standard attack, with all modifiers. If your roll exceeds your attacker's roll, you successfully defend the attack and strike your attacker with your counter, and may roll for damage and inflict the specified effect.

Crooked Strike

You strike to your opponent's hands. Your target must make a Dexterity Save vs. your DC, or their next attack will come at a Disadvantage. When used offensively, add your Superiority Dice to the attack roll.

Strike of Wrath

The Strike of Wrath is an overpowering blow from above, which Criticals on a roll of 19-20. When used offensively, add your Superiority Dice result to your damage rolls. When used defensively, calculate damage as normal.

Vor

At 3rd Level, whenever you have Initiative over the target you're currently attacking, you make your attack at an Advantage. If you already have Advantage because of another skill or ability, you may reroll the lowest dice result.

Agile Swordsman

At 7th Level, the Master of the Longsword is adept at using leverage rather than strength to maneuver their weapon. Thus, when wielding a longsword in two hands, you may treat it as a Finesse weapon so long as the creature is of Medium size or larger.

Indes

At 10th Level, when you perform a successful Master Strike to defend yourself from an attack, you may expend a Superiority Dice to take Initiative away from that opponent. Reroll for Initiative with that creature, adding the results of your Superiority Dice to the roll. If successful, whenever that creature moves or performs an Action, you may opt to take any Action first. If you do take an Action, you sacrifice your normal turn on that round, and may not use any movement points that round. If you have any Reactions remaining that turn, you may utilize them as normal.

Whether or not you take an Action before that creature, this condition remains in effect either until you attack a different creature, or the creature you are attacking is killed or otherwise removed from combat.

Provost of Arms

At 10th Level, your superiority dice turn into d10s. You also gain two additional Master Strikes.

Parting Strike

You step back from your opponent and strike at the top of his scalp. On a successful hit, they must make a Constitution Save vs. your DC or be blinded for the next turn. When used as an offensive strike, add the results of your Superiority Dice to the attack roll.

Squinting Strike

Offensive

When you make an unsuccessful melee attack, you may expend one Superiority Dice to position your sword between you and your opponent to cover the line. You gain a free Opportunity Attack against that target's next action — even if the target is immune or the action is not one that normally provokes an Opportunity Attack — and may add the results of your Superiority Dice to the attack roll. The damage type is considered Piercing for this attack.

Defensive

You displace your opponent's attack then thrust at his face, inflicting Piercing damage. Your attacker must make a Constitution Save vs. your DC or be Distressed.

Deft Defense

At 15th Level, when wielding a longsword in two hands, you may add half your Proficiency (rounded down) to AC against the last target you attacked.

Overwhelming Barrage

At 15th Level, the Master of the Longsword is able to overwhelm their opponent with a rapid tempo of attack. When they have Initiative over their current target target, after making a Melee Attack they may use a Bonus Action to follow up with another Melee Attack. This also applies to the Fighter's extra attacks at levels 5, 11, and 20, for a potential of 8 total attacks. As with Two-Weapon Fighting, the Ability modifier is not added to the damage roll.

Master of Arms

At 18th Level, your Superiorty Dice improve to 3d12, and gain a final Master Strike.

Thwarting Strike

Offensive

When you make an unsuccessful melee attack, you may expend one Superiority Dice to make a Bonus Attack striking laterally around the other side at your opponent's head. Add your Superiority dice to the attack roll.

Defensive

You counter your opponent's strike with a high, level cut at his head and block his line of attack. His next attack comes at a Disadvantage.