Swordmaster

The sword has a special place among all common weapons - daggers, axes and hammers are all tools, bows and spears are often used for hunting, maces and clubs are nothing more than glorified sticks. The sword has many varieties, but they all serve one purpose alone - killing. Those who seek to emulate such single minded dedication and transcend it even further, often follow in the tradition of the Swordmaster - a legendary warrior who once cut the planet in half.

Sword Mastery

Your dedication to the blade grants you unparalleled versatility. From level 3, when you wield a longsword*, it has the Finesse property for you. Also, you can choose which damage type it deals from slashing, piercing and bludgeoning. Starting at level 10, you can also choose force.

*remember, while the weapon might mechanically be a longsword, lore-wise your character can wield whatever you desire, as long as your DM agrees

Killing Arts

At the 3rd level you become learned in the lethal arts. Once per short rest you can use your bonus action to attack with a longsword, adding your proficiency bonus (or the corresponding proficiency die) to the damage. You can also select one of the Arts bellow to use with that strike. You can use this feature again if, while using a longsword, you reduce a creature to 0 hit points or score a critical hit.

Saving Throws. Some of your killing arts require your target to make a saving throw to resist the art's effects. The saving throw DC is calculated as follows:

Killing Art DC = 8 + your proficiency bonus + your Strength or Dexterity modifier (your choice)

Overplay Commitment Distract an enemy with a weaker attack as a setup for more devastating blows. You do not add your ability modifier to this attack's damage, but if you hit, all other attacks you make against this enemy are made at an advantage until the end of your turn.
Underplay Commitment Feign a weak, probing attack while making a powerful strike to disturb the enemy. When you hit a creature with the additional attack, the target must make a Strength saving throw and on a failed save, it suffers disadvantage on all attack made until the start of your next turn.
Advantageous Position When you make the additional attack, you can direct the target into a position advantageous to your allies. The next attack roll against it by an attacker other than you has advantage if the attack is made before the start of your next turn.
Hold Position When you hit a creature with the additional attack, you can attempt to force it to maintain position. The target must make a Wisdom saving throw and on a failed save, it has its speed reduced to 0 and cannot take reactions until the start of your next turn.
Backward Momentum When you hit a creature with the additional attack, you encourage them to keep attacking you. The target must make a Wisdom saving throw and it fails, you can make an opportunity attack against it, if it attacks any creature other than you before the start of your next turn.
Forward Momentum When you hit a creature with the additional attack, you can attempt to drive the target back. If the target is Large or smaller, it must make a Strength saving throw. On a failed save, you push the target up to 15 feet away from you. You can also move up to 10 feet towards it.
Disturb Footwork When you hit a creature with the additional attack, you can attempt to knock the enemy off their stride. If the target is Large or smaller, it must make a Dexterity saving throw. On a failed save, you knock the target prone.
Evasive Footwork Maintain light and evasive footwork. If you hit, add your proficiency bonus (or the corresponding proficiency die) to your AC until the start of your next turn.

Understanding

With the progress you make on the path of the blade comes understanding, not only of the weapon and the enemies you wield it against but also yourself and those who came before you. At level 7 you gain proficiency in History and a set of abilities involving your sword. First, the damage dice for your longsword cannot be lower than a d10. Second If you wield a longsword with two hands, you gain a +2 bonus to your AC as if you were using a shield. Third, you recharge your Killing Arts whenever you use your Action Surge. And finally, you learn two more Killing Arts.

Judgement Clash. Regardless of whether you hit, the creature you targeted must make a Charisma saving throw. If it fails, you learn its alignment, but if it succeeds, it can make a Charisma (Deception) check against the same DC to falsify the result, instead of simply not giving it. You can choose to not deal damage with this attack after learning the result. You cannot use this ability more than once on the same creature.
Revealing Clash You exchange a series of quick clashes with a creature in an attempt to show your character. This informs your opponent of your alignment and attitude towards it regardless of whether you speak its language. You can make a Charisma (Persuasion) check at advantage, or a Charisma (Deception) normally, if you were attempting to lie. This attack deals no damage, but is more convincing than simply talking.

To Cut

To use a sword is to cut, and only a third-rate swordsman is limited by the length of their blade. At level 10 your strikes can now deal force damage and you learn two new Killing Arts.

Acausal Slash You align your sword and the target within your perception. The additional attack has base range of 60 feet. You can target creatures behind full cover, as long as you know their precise location. In that case, the attack roll is made at a disadvantage and you do not add your proficiency bonus to the attack's damage.
Horizontal Slash The swing of your sword cuts all you can see. Choose a direction and elevation above the ground. Every creature within a 20ft cone in front of you at your chosen elevation must make a Dexterity saving throw. If it fails, it takes damage equal to your normal Killing Art damage. This art needs to be made with the main action and does not benefit from Extra Attack or any similar ability.

Marking Cut

From 15th level you no longer need to kill a creature by yourself to continue your dance of death. Each creature you damage with a melee weapon attack is marked for 10 minutes. When you see a marked creature die, you regain the use of your Killing Arts.

True Kill

If you kill a creature with your own hands and still have an attack left to make, you can use it to perform an extra Killing Art. You add no bonus damage and can only utilize options that would be possible with the bonus action attack. This feature can only be used once per turn.

Sword Saint

There is nothing you cannot cut, not even magic is beyond your reach. At level 18 you get new Killing Arts that allows you break just that.

Concentration Break. If you hit a concentrating creature with the additional attack, you can attempt to cut their mind directly. Its Concentration Save DC cannot be lower that your Killing Arts DC. If it fails, it cannot maintain any effect that requires concentration. The creature can repeat the saving throw on each of its turns to end the effect.
Magic Break. You can make an attack roll against a magical effect to immediately end it on hit. The AC of the effect is equal to 10 + twice the spell's level. You can do the same as a reaction, when you see the casting of a spell that would directly affect you. Since destroying magic counts as killing it, you regain the use of your Killing Arts if you succeed.

Shortsword of Light

At the pinnacle of mortal ability, you learned the technique the Swordmaster deemed a failure. And while the devastation it brings cannot compare to the success, it is not to be underestimated.

If you have not used any actions or bonus action on your turn and have a Killing Art ready, you can spend them all to swing your sword at the speed of light and unleash this devastating attack. Every object, creature, terrain and phenomenon in both Material and Ethereal planes in a 200ft line in front of you must make a Dexterity saving throw, taking 12d12 + 60 force damage on failure and half that on success. You than suffer three levels of Exhaustion.