The Master of the Longsword is a warrior who focuses his 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 he lacks in his ability to engage multiple threats he makes up for in his ability to overwhelm and dominate a single target.
 
However, because his focus and energy is on mastery of his weapon to its fullest extent, the Master of the Longsword is initially unable to utilize these techniques with any other weapon. Only later in his training and study is he able to learn to apply his ability to certain other two-handed arms.

Restriction

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

Master of the Longsword

At 3rd Level, the Master of the Longsword is adept at using leverage rather than strength to maneuver his weapon. Thus, when wielding a longsword in two-hands, you may treat it as a Finesse weapon.

Additionally, you gain four Superiority Dice (1d8) which you may expend on performing one of a number of techniques from the class' maneuvers list. The available maneuvers are determined by your fighting style (see below). Save DC = 8 + your Proficiency Bonus + your Dexterity modifier.

Chasing After

If your opponent makes an unsuccessful melee attack against you, you may expend one Superiority Dice to gain an Attack of Opportunity against him. Add your Superiority Dice to the attack roll.

Coup de Grace

When striking a Prone opponent, you may expend one Superiority Dice to deliver a devastating blow. On a regular hit, roll twice for damage, and add the result of both to the result of your Superiority Dice. On a Critical Hit, roll for damage three times, and add the results to your Superiority Dice. If Half-Sword is enabled, roll three times on a regular hit and four times on a critical hit.

Crooked Strike (Offense)

When you make a succesful melee attack, you may expend one Superiority Dice to strike to your opponent's hands. Your target must make a Save vs. your DC, or suffer an AB penalty equal to your Superiority Dice result on their next attack.

Doubling

When you make an unsuccessful melee attack, you may expend one Superiority Dice to attempt a second attack, adding your Superiority Dice to the attack roll. If this second attack is successful, you inflict 1d4 + any modifiers Slashing damage.

Half-Sword

When this mode is active, you take your sword by the blade, shortening up your reach and reducing power but improving your control over your weapon. All standard attacks with your weapon while enabled inflict 2d4 Piercing damage, however you may now critical on an unmodified roll of 19 or above.

Indes

When you make a successful melee attack against a target who has Initiative on you, you can expend one Superiority Dice to force a reroll for Initiative against that target. Add the result of your Superiority Dice to your roll. This applies only against that target, until it is either defeated or you attack a different target with a Standard Attack.

Mordhau

When you make a successful weapon attack, you may expend one Superiority Dice to take your sword by the blade, and strike with the hiltl instead of your blade. Add your Superiority Dice to the damage roll, and calculate the damage as Bludgeoning. Additionally, your opponent must make a Constitution Save vs. your DC, or be stunned for the next round.

Parting Strike (Offense)

When you make a successful melee attack, you may expend one Superiority Dice to cut to the top of your opponent's scalp. Your opponent must make a Save vs. your DC or be blinded for a number of rounds equal to your Superiority Dice result.

Squinting Strike (Offense)

When you make an unsuccesful melee attack, you may expend one Superiority Dice to position your sword between you and your opponent to cover the line. You gain an automatic 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.

Strike of Wrath (Offense)

When you make a successful melee attack, you may expend one Superiority Dice to make an overpowering blow from above. Add your Superiority Dice roll to your damage roll, and your target must make a Save vs. your DC or suffer 2 points of CON damage against that attack. If the target's adjusted HP against the attack would result in fatal damage, the target is dead even if the attack would not be fatal once the CON damage is restored.

Thwarting Strike (Offense)

When you make an unsuccessful melee attack, you may expend one Superiority Dice to strike laterally around your opponent's defenses to the opposite side on your next attack. Your next attack against the target has Advantage, and adds the result of your Superiority Dice to the attack roll.

Transmuting

When you make an unsuccessful melee attack, you may expend one Superiority Dice to attempt a second attack, adding your Superiorty Dice to the attack roll. If this second attack is successful, you inflict 1d4 + any modifiers Piercing damage.

Winding

When you make an unsuccessful melee attack, you may expend one Superiority Dice to gauge your opponent's intentions. Make a Perception check modified by your Superiority Dice vs. a DC of 8 + your opponent's Proficiency Bonus and the Ability bonus they use to attack. On a successful check, you read your opponent and gain Advantage on your next Standard Attack. However on failure, you misread your opponent and make your next attack at a Disadvantage.

Wrestling At Sword

When you make a successful weapon attack, you may expend one Superiority Dice to grapple your target, using your weapon as leverage rather than inflicting damage. Make an Athletics check modified by your Superiority Dice, plus half your Proficiency Bonus and any attack bonus on your weapon, against your opponent's Athletics or Acrobatics check. If your grapple is successful, your opponent is knocked Prone.

Fighting Style

At 3rd Level you may choose a particular fighting style based on you armor type:    

Bare Fighting:    When wearing light or no armor, add half your Proficiency Bonus (rounded down) as AC against the last target you attacked.
Harness Fighting:    When wearing medium or heavy armor, treat your armor as one category lighter when calculating your Dexterity bonus to AC against the last target you attacked.

Bare Fighting

When wearing light or no armor, add half your Proficiency Bonus (rounded down) as AC against the last target you attacked. You also gain access to these abilities:

Indes
Strike of Wrath (Offense)
Crooked Strike (Offense)
Thwarting Strike (Offense)
Parting Strike (Offense)
Squinting Strike (Offense)
Winding
Doubling
Transmuting
Chasing After

Harness Fighting

When wearing medium or heavy armor, treat your armor as one category lighter when calculating your Dexterity bonus to AC against the last target you attacked. It also grants you access to these abilities:

Indes
Strike of Wrath (Offense)
Crooked Strike (Offense)
Thwarting Strike (Offense)
Parting Strike (Offense)
Squinting Strike (Offense)
Mordhau
Half-Sword
Coup de Grace
Wrestling at Sword

Master Strike

At 7th Level, if the last target you attacked with a melee weapon makes a successful standard attack against you with a melee weapon, and you are not threatened by another enemy, you may use a Reaction to execute one of the Master Strike maneuvers to counter your opponent's attack.

Crooked Strike (Defense)

You counter by striking to your opponent's hands, inflicting 1d6 + any modifiers Slashing damage. Your target must make a Dexterity Save vs. your DC, or their next attack will come at a disadvantage.

Parting Strike (Defense)

You step back from your opponent's attack, and retaliate with a cut to the top of their scalp, inflicting 1d6 + any modifiers Slashing damage. They must make a Constitution Save vs. your DC or be blinded for their next turn.

Squinting Strike (Defense)

You displace your opponent's attack then thrust at his face, inflicting 1d6 + any modifiers Piercing damage. Your attacker must make a Constitution Save vs. your DC or be Distressed.

Strike of Wrath (Defense)

You intercept your opponent's strike with a powerful blow from above, inflicting 1d6 + any modifiers Slashing Damage. Your target must make a Constitution Save vs. your DC, or suffer 2 points of CON damage against that attack. If the target's adjusted HP against the attack would result in fatal damage, the target is dead even if the attack would not be fatal once the CON damage is restored.

Thwarting Strike (Defense)

You counter your opponent's strike with a high, level cut at his head, inflicting 1d6 + any modifiers Slashing damage. This counter ignores immunity to Critical Hits granted by your opponent's equipment.

Vor

At 7th Level, gain +2 AB and AC if you have initiative over your current target.

Improved Combat Superiority

At 10th Level, your superiority dice turn into d10s. At 18th Level, they turn into d12s.

Deft Defense

At 15th Level, when wielding a longsword in two hands, add any Enchantment Bonus on the weapon to your AC.

Weapon Synergy

At 18th Level, you may now use your abilities with the following weapons: Quarterstaff, Spear, Halberd, so long as it is wielded in two hands.