Myrmidon

A myrmidon is a warrior who has eschewed training with all other weapons and even the restriction of heavier forms of armor to achieve an almost unparalleled mastery with their sword. This, however, leaves their defenses somewhat lacking, forcing them to develop an equally formidable speed and battlefield awareness to evade incoming blows. 

While myrmidons can deal with flimsier foes with ease, that same prowess may not necessarily be replicated against sturdier enemies. When overwhelmed by too many of such enemies, they could potentially lose their superior edge and be felled.

Duelist's Blow

Starting at 3rd level, your battlefield instinct gives allows you to ambush opponents easier. You can give yourself a bonus to your initiative rolls equal to your Wisdom modifier (a minimum of +1).

You also learn the hunter's mark spell, and Wisdom is your spellcasting ability for it. You can use it a number of times equal to your wisdom modifier (a minimum of once) without expending a spell slot. You regain all expended uses when you finish a long rest. You don't need advantage on the attack roll to use your Sneak Attack on creatures marked by this spell if you are within 5 feet of it and you don't have disadvantage on the attack roll. All the other rules for Sneak Attack still apply to you.

Vantage

Your keen mind can detect incoming attacks and react accordingly. Starting when you choose this archetype at 3rd level, when a creature within 5 feet of you makes an attack roll on you, you can use your reaction to make an attack roll with a finesse weapon you are currently holding. If the result is higher than the attack roll of the creature, you can make an attack on that creature first. If the result is lower than the creature's attack roll, nothing happens and you do not expend a usage of this feature. This does not prevent the creature from making its attack unless your attack reduces the creature's hit points to 0.

You may use this feature a number of times equal to your Wisdom modifier (minimum of 1). You regain all expended uses after a short or long rest.

Continue

Beginning at 9th level, if you successfully perform a sneak attack or roll a critical hit on an attack roll, you can make another attack or cast another spell as part of the same action. You must target the same creature, and you must attack with the same weapon or spell you attacked with the first time. You may keep making attacks as long as you keep rolling critical hits.

Galeforce

Your flowing use of your weapon allows you to enter battle and strike more quickly. Beginning at 13th level, when you land a killing blow, your speed is doubled until the end of your turn. Additionally, your walking speed increases by 5 and whenever you roll initiative, your result increases by 1d4.

Improved Critical

Starting at 17th level, your weapon attacks score a critical hit on a roll of 19 or 20.

Astra

Starting at 17th level, you learn a legendary sword technique, allowing you to attack multiple targets at high speed. You can use your action to attack up to five creatures you can see within 30 feet with a finesse weapon simultaneously. Make a melee weapon attack against each target. You can use your sneak attack on all targets of this attack. You count as within 5 feet of the creature for the purposes of Sneak Attack. All other rules for Sneak Attack still apply.

After attacking, you appear in an unoccupied space you can see within 5 feet of one of the targets you hit or missed. You can use Continue afterwards, but only if you appeared within 5 feet of a target you hit with a sneak attack or critical.

You may use this feature a number of times equal to your Wisdom modifier (minimum of 1). You regain all expended uses after a long rest.