The Riposte skill will require 50 points spent in one of either the Might or Finesse class specializations to unlock the final Weaponry focus, which will unlock the after-parry attack for the specified weapons. It is the leftmost tree in each of those classes. Note that the Sorcery tree does not have an after-parry special attack! Alternatively, you can put 37 points into each of the three trees to unlock the "Mastery of Arms" perk which will give you every available weapon ability.
To perform the special attack, follow the method to parry found in the "Foiled Again!" achievement, then simply use a basic attack with whichever Weaponry focus you've obtained during the "slow-mo" time after the successful parry.