This mechanic arose naturally in game the other day in my Mutant Future campaign. The group has wandered into an alternate reality/different dimension. They are staying in an elven castle and managed to get on the wrong side of the prince at the banquet that was thrown in the party's honor the night they arrived. The subject of swordplay came up and the prince challenged party ladies man James Bomb to a friendly fencing match after dinner. Several other characters leapt to Mr. Bombs defense, offering to take his place in the practice chamber. Fencing with the prince after dinner proved educational indeed, as he was a dazzling swordsman. Still, the second character to face him did manage to score a hit and win honor for the party!
These are the simple rules I use to simulate a master fencers ability to attack in flurries while parrying an opponent's strikes.
A parry is simply the forfeiture of your normal attack - this gains you the right to attack your opponent's blade if you are attacked in the following round. The parry attempt is a normal attack roll thrown against the attacker's AC, modified by dexterity and not strength. A success indicates that the attack was parried away harmlessly. You have to state which opponent is going to be the recipient of your parry when you forfeit your attack roll.
This rule is intended to replace any other weapon specialization proficiencies or feats used. Choosing a weapon as your focus and pursuing the road toward mastering it requires dedicated practice with the weapon.
Weapon Focus: After gaining a level of experience, if a character dedicates her training to a single weapon, weapon mastery can be attained. The character gains one extra attack per round with the chosen weapon. The character receives a -2 penalty to attack rolls with all other weapons.
Some follow the pursuit of perfection with a single weapon to its utmost, almost completely neglecting all other forms of combat.
Weapon Mastery: After gaining a level of experience, a character that already has a weapon focus may seek to devote herself completely to the craft of fighting with her chosen weapon. The character gains one extra attack per round with her chosen weapon (two extra attacks total for focus and mastery). The character may choose to use her dexterity modifier rather than her strength modifier if the chosen weapon is light (shortsword or lighter, in general) or her constitution modifier instead of her strength modifier if the chosen weapon is heavy. The character now receives a cumulative total of a -6 penalty to attack rolls with all other weapons.
The Master Fencer - lunge, parry, riposte!
Combining the Weapon Mastery and Parry rules, it is easy to make an effective and deadly fencer. Three attacks a round, modified by dexterity (an otherworldly 20 in the case of the hot blooded elven prince), with the ability to sacrifice one or two attacks to parry the futile attempts of my brave Mutant Future players... That prince was a bad-ass. The practice duels were to "first blood", which is really a fancy name for the first person to land a damaging hit wins the duel! The first character to face him went down in a flurry of rapier strikes, his only attempted attack beaten away effortlessly.