True blade

This subclass is inspired by Vergil from devil may cry, but doesn't try to copy it.

Through some way or the other, you aquired a technique with which you can cut through reality itself, potentially even shattering it. Part of this technique is speed, dextrerity and discipline. How you found these powers is absolutely up to you, but I have an example, because that's the explanation in the campaign I'm playtesting it:

You are part of an order that protects the universe from a threat from outside reality ( probably the far realm, some abberations). These creatures use a power from their existence that destroys reality itself. But while normal attacks are mostly ineffective, because they are so different from this reality that they aren't affected by the things that would normally harm someone, they themselves are vulnerable to this power, because it is a power from their realm. So you and the order fight fire with fire and use their power against them.

A thousand cuts

Beginning with level 3, you have mastered the use of your weapon to such a degree that you can fight off multiple enemies at once. When you take the attack action you can replace one of the attacks with this feature. If you do, choose any number of creatures within 5 feet of you. A chosen creature has to make a dexterity saving throw ( DC: 8+ your proficiency Bonus + your dexterity modifier ). On a failed save it takes a number of d6 equal to your proficiency Bonus, plus your dexterity modifier (PB d6 + dex) of magical damage, or half as much on a successful save. The damage type is one of the damage types of the weapon you're holding. 

You can use this feature a number of times equal to your proficiency Bonus and you regain any spent uses when you finish a long rest.

Instant assail

Starting with 7th Level, you can use your Bonus action to teleport to an unoccupied space you can see within 30 feet. After you did, you may make a melee weapon attack against a creature within range, but the attack deals force damage instead.

You can use this feature a number of times equal to your charisma modifier, and you regain any spent uses when you finish a long rest. 

True blade

Beginning with level 10, whenever you deal damage with a melee weapon attack, you can replace the damage type to force damage. Additionally, all your attacks count as magical.

Shattering strikes

At 15th level you are powerful enough to shatter reality itself with your strongest strikes.

You score a critical hit when you roll a natural 19 or 20.

When you score a critical hit with a melee weapon attack, you deal an extra 2d6 force damage.

Ultimate technique

Starting at Level 18, you can unleash your full power to assail your foes with multiple devastating strikes.

You can use your action to make up to 4 melee weapon attacks against every creature within a 15 foot radius centered around a point you can see within 10 feet of you. You can add your charisma modifier to the roll in addition to any other modifiers. You deal an additional 1d6 force damage and an additional 1d6 force damage for every time you've hit this creature with this ability this turn. Any creature that is killed with this ability is hit out of reality. It doesn't leave a body behind (altough it leaves anything it had worn or carried) and can't be brought back with anything other than a wish spell. While resolving this action, choose in which order you attack which target. As part of the action, before resolving the attacks of the next target, you move within reach. This movement doesn't count as your movement and doesn't provoke opportunity attacks, and you can choose to teleport instead. After resolving all the attacks, you can move the same was as above to an unoccupied space anywhere within the Radius. 

You can use this feature once and regain its use after 1d4 days.