Dekko's Gambit

"Dekko's Gambit is a crowd pleaser, that's for sure. Instead of blocking with the blade, one blocks with their vambrace or gauntlet, with the blade in descending guard. This allows an immediate counter attack that can best be described as 'cruel' or 'hideous'. If delivered just right, the blade draw-cuts up the torso from the diaphragm to the throat, then lashes up in a leveraged attack that delivers a powerstroke to the enemy's chin. I have seen the maneuver used twice, both times by its inventor, and both times the blade split the head in half. Both times the crowd witnessing the duel broke out in horror and cheers."
~Seak Beardo Von Shmit, of Abide and Seak's Bizarre Bazaar

Dekko's Gambit

Very Rare
Dekko's Gambit takes one month to learn. It requires a two handed (or versatile) slashing weapon.
As a bonus action, you can declare that you go into "Dekko's Stance." If an enemy attacks you until the beginning of your next turn and misses, you may use your reaction to counterattack. If this attack hits, it automatically does critical damage, and if the enemy has half your HD or lower their HP drops to 0 and they begin dying from disembowelment and head trauma. Any successful attack made against you between your counterattack and the beginning of your next turn is automatically a critical hit.