Base Class: Rogue
Vorpal rogues are few and mighty. They attempt to take their lethality to the highest of peaks, as well as their dexterity. Rogues of this following give up any and all defense they have against enemies to provide overwhelming offensive capability. So much so that rarely does their frailty matter.
But there isn't so few of them for no reason.
Level 3: Fragile Might
While your armor doesn't get in your way, you have been able to use full range of motion for your strikes, making them deadlier, and more precise.
your Armor Class does not include your dexterity modifier. This includes your unarmored Armor Class. as well as any Armor Class bonuses that may include the dexterity modifier.
For every two points of Armor Class below 15 (Ex. 13=1. 11=2) You gain one additional sneak attack die (d6).
Additional dice from your Fragile Might subclass feature will only be calculated if the sneak attack you are adding the dice to is from melee range.
Level 3: True Dexterity
You’ve realized that sometimes armor is more weight than it's worth.
For every two points of Armor Class below 15. You gain one additional Dexterity Ability Score Point. This feature cannot increase your dexterity ability score above 20.
Level 13: White Nail
If your armor class is less than 15. you gain the following ability.
Whenever you score a critical hit against an enemy, you may add your sneak attack damage to it, if you weren't able to before. This includes any additional dice from your Fragile Might class feature.
If you were already able to add your sneak attack damage prior to receiving a critical hit, you may instead inflict a Grievous Wound on the creature.
A Grievous Wound will immediately kill a creature upon its health becoming a value equal to or less than your total dexterity ability score.
Level 9: Lithe
Realizing what a useful instrument one's own body can be when unimpeded, you have become more agile than ever before.
If your Armor Class is lower than 15, You gain the following abilities.
Your size counts as one size smaller. unless you are calculating carry, push, lift, or pull weight, or jump height.
You make no additional noise when taking any action.
Level 17: Unarmored Mastery
You've managed to achieve the apex of nimbleness and deadliness at the cost of vulnerability.
Your class features Fragile Might and True Dexterity now both activate for every one point of Armor Class below 15, instead of every two points of Armor Class below 15.
additionally, When reducing an enemy to zero hitpoints you may decrease your armor class by one point, this lasts until the end of combat.