Blade Dancer

The Blade Dancer archetype excels at inserting itself into the thick of combat, and reacting swiftly to alter and revel in it's flow.
At third level, when you take this Archetype you gain the Sudden Riposte and Dance of the Nimble Features. 
Sudden Riposte allows you, when an enemy attacks you with a melee strike that misses, to make a weapon attack of your own. This weapon attack requires no action or reaction to initiate, but does not receive the benefits of a critical hit. When you reach Ninth Level, attacks made with the Sudden Riposte feature are considered Sneak Attacks. You can only make a Sudden Riposte a number of times equal to your Dexterity Modifier and you regain all spent uses at the end of a Short or Long Rest. 
Dance of the Nimble allows you enhanced methods of moving through the fray; Whenever you use your Cunning Action to Dash or Disengage, the amount of distance you can take as a part of that action increases by 10 feet. Additionally, you can now find pockets of stillness among the carnage and use your Cunning Action to Dodge. 

At 9th level, you develop a series of unorthodox techniques to give you the edge over the more honourable sort. The Dirty Tactics feature allows you choose one of a list of techniques at the end of a long rest. Dirty Tactics confer bonuses onto yourself such as temporary hit points and an AC increase, or debuffs onto your opponents such as the Poisoned or Blinded conditions whenever you make an attack with your Sudden Riposte. To activate the effect of a Dirty Tactic, you must choose a number of Sneak Attack Die (the number of which is described in the Tactics list) and discount them from your damage roll. When you reach the 13th level you can prepare a second Dirty Tactic.

At 13th level, you gain the Harrier feature, this allows to take your invasive, haranguing nature to a new height, as now, whenever an enemy within 5 feet of you makes an attack against a creature that isn't you, you can use your Sudden Riposte on them. Additionally, when an ally successfully attacks an enemy under the influence of one of your Dirty Tactics, you can use your reaction to regain 2d6 Hit Points. Finally, you regain a use of Sudden Riposte whenever you score a critical hit against an enemy.

At 17th level you gain the benefits of Denouement, the finality of fatality. Whenever you reduce a target to 0 Hit Points, you gain advantage on your next weapon attack.
Your ability to exploit the openings of your enemy's defense has moved beyond the reactionary, as well. Now, whenever you make a Melee Strike that satisfies the criteria of Sneak Attack, you can use a Dirty Tactic without expending a use of your Sudden Riposte. 

Sudden Riposte

When you choose this archetype at 3rd level, you learn to exploit every opening in an opponents defense, especially the ones that appear in the moments after they strike. At any point, while you are holding a finesse weapon, and you are targeted by a melee strike that misses, you can make a melee weapon attack of your own against that creature, requiring no action. You can use this feature a number of times equal to your Dexterity modifier per short or long rest and, beginning at level 9, is considered a sneak attack. However, this attack does not receive the benefits of a Critical Success.

Dance of the Nimble.

Your knack for weaving through the tangle of combat has surpassed friend and foe, alike. Starting at 3rd level, whenever you use your Cunning Action to Dash or Disengage, you can add 10 ft to your speed for that action. A creature with a speed of 30ft, when dashing, would be able to travel 70ft or, when disengaging, have a speed of 40ft, for example. Additionally, you've learned to fall into the still spots amid the thrash and carnage around you, and can now use your Cunning Action to Dodge. 

Dirty Tactics

Simply being lithe and dexterous will not always be enough to keep you safe, you've come to know. Starting at 9th level you begin to develop some duplicitous techniques to punish those who would harm you. At the end of a Long Rest choose one Dirty Tactic from the options listed; until the end of your next long rest that effect applies to every attack you make as part of your Sudden Riposte.
At 13th Level, you can prepare a second Dirty Tactic.

Combatant Collapse

Strike low. When using your Sudden Riposte, you can elect to forgo 2 of your sneak attack die, to instead force the target to make a Strength Save with a DC equal to 8+your attack bonus. On a failed save the target is knocked prone. You can choose to expend a third sneak attack die to add 1d6 to the save DC.

Forced Disarmament

The best opponent is one that is not armed and stands at the business end of your blade. When you make your Sudden Riposte attack, you can elect not to roll 2 of your Sneak Attack die and, instead, strike your foe in the hand, forcing them to drop whatever it is holding. You can also choose to neglect 2 more Sneak Attack die to slide the object up to 20ft away from your Target, assuming the object in question weighs less than 10lbs. 

Impervious

When embroiled in the back and forth of the dance of death, you become even harder to hit, when using your Sudden Riposte, you can elect to ignore a number of your sneak attack die no greater than your half of your total number of Sneak attack die, rounded down. You can then take that number and add it onto your AC. This bonus lasts until the next time your are hit by an attack roll.

Obfuscation

Not every weapon need be ornate and elegant. Dirt is just as good. When you attack with Sudden Riposte, you can choose not to roll 2 of your sneak attack die and, instead, throw some type of common filth, like dirt, sand, snow, mud or... worse into their eyes, rendering them blind if they fail a constitution save with a DC of 8+your attack bonus until they take an action to clear the grit from their eyes. At the cost of a third sneak attack die, you can increase the DC by 1d6.

Vigor Victimize

Inflicting your martial prowess on those who stand against you fills you with vigor. Whenever you successfully make an attack with your Sudden Riposte you can elect not to roll as many sneak attack die as you wish. In exchange you gain temporary hit points equal to that number of d4 plus your constitution modifier.

Viper's Venom

You have a supply of venom that you can draw out of your weapon in a pinch. Whenever you use your Sudden Riposte, you can elect not to roll 2 or more of your sneak attack Die. When you do this your target becomes poisoned and, every round when you start your turn, it takes a d4 Poison Damage for every sneak attack die you expended. The Poisoning ends when the target makes a Constitution saving throw with a DC of 8+Your Attack Modifier which it can make at the end of your turn. 

Harrier

Over time, you've learned to weave your companions and allies into your battleground dance. Beginning at 13th level you gain the following benefits:
Whenever an Enemy within 5 feet of you makes an attack against a creature that isn't you, you can target them with your Sudden Riposte.
Whenever you score a critical hit, you regain 1 use of your Sudden Riposte. 
Whenever an Ally makes a successful attack roll against a creature that is under the effect of one of your Dirty Tactics, you can use your reaction to regain Hit Points equal to 2d6 + your Constitution Modifier.  

Denouement

As your skills approach their apex, you gain an air of finality in combat; when you make the decisive final strike against a creature and reduce it to 0 hit points, you can carry that momentum into your next engagement and gain advantage on your next Weapon Attack. 
Additionally, you've become so adept at bending the rules of engagement, you are no longer dependent on your foe's erroneous act. Now, whenever you make a weapon attack that satisfies the requirements of Sneak Attack, you can use a Dirty Tactic as part of that attack. This Dirty Tactic does not count as a use of your Sudden Riposte.