Parrying, evading and weapon locks.

Parrying:
When it is your reaction turn, you are free to attempt to parry an attack if you have a melee weapon drawn. You can only parry melee attacks, unless you have the foresight power. When attempting to parry, your parry score plus dexterity modifier is taken away from the opponents hit score. If this takes their score lower than the needed to hit score, the parry is successful and you take no damage. If a parry takes the opponent down to the exact number they needed to hit, then a weapon lock is engaged. If a parry score takes an opponents hit score down to 0, the parrying character gets a free counter attack with the parrying weapon.

Evading:
Exactly the same as Parrying but with a few differences: 1) There is no weapon lock, the opponent scores a hit if they roll the exact number they needed. 2) There is no free counter attack gained. 3) You don't need a melee weapon drawn to evade. 4) The evader uses their evade skill plus their dexterity bonus.

Weapon lock:
When a weapon lock ensues, both combatants roll 1d20 and add their strength modifier (or dexterity modifier if the combatant has weapon finesse). The one who rolls higher gains the following benefit.
Attacker: The attacker gains 1 additional attack, one of the attackers attacks auto-hits. The other(s) must be rolled for.
Defender: The attacker's attack is negated and the defender may take a free counter attack, rolls to hit are made as usual on this free attack.