When you use a melee weapon or fight unarmed you can use the parry skill to enter a defensive stance. This allows you to block melee attacks and make counter-attacks against less experienced opponents.
Classes: Barbarian, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Swashbuckler; Arcane trickster, Assassin, Blackguard, Divine champion, Duelist, Dwarven defender, Eldritch knight, Frenzied berserker, Invisible blade, Red dragon disciple, Shadow Thief of Amn, Shadowdancer, Warpriest, Weapon master
Requires training: No
Check: The DC is the modified attack roll of the incoming blow. A successful Parry check means that the attack does not damage the parrying character. A character may only parry a number of attacks per round equal to the number of attacks per round available to the character.
Special: If the parry is successful and the difference between the roll and the DC is ten points or greater, a counterattack occurs. A counterattack is a free attack made by the parrying character against the parried opponent. The first counterattack in a round is made with a full attack bonus and subsequent counterattacks are made at -5, -10, -15, and so on.
Use: Select the Parry mode. The character will remain in Parry mode until the mode is exited.)
For arguments sake and to make it all easy to calculate, take for example two characters:
Parry skill user: ranger 20 (DEX oriented)
Attacker: fighter 20 (STR oriented)
Parry skill roll = 1d20 + 23 (maxed parry skill) + DEX mod - 2 (armor check penalty )
Parry DC = incoming blow = 1d20 + 20(BAB of fighter) + 4(weapon) +2 (Weapon focus feats) + STR mod
Assuming that the fighter's STR mod is equivalent to the DEX mod of the ranger and that they both roll exactly the same on the d20, a character with maxed parry will avoid being hit 50+% of the time, which is balanced (Note: Not statistically correct - in the above example the parry adjustments are at -5 with equivalent ability modifiers compared to the incoming blow adjustments and the parry would only be successful 30% of the time; if the parry and incoming blow have the same adjustments the parry will have a 52.5% success rate). The AB of the ranger in this example is not added to the parry check.
If the parry check exceeds the DC by 10 (5 with Improved parry), a riposte attack opportunity is triggered. This does not result in an automatic hit - instead, one attack at the highest BAB is made, which can miss like any other normal attack. Only the first riposte attack is made at max AB; the ones following take cumulative -5 penalties.
If a parry check is failed, it also does not result in an automatic hit. The attacker's blow is then compared to the parrying character's AC normally.
Using a duelist instead of a ranger results in different calculations: at level 7, the duelist will add his level to the Parry skill use. Also, the duelist will not typically have an armor check penalty. This increases the parry roll by 9 points, tipping the balance in the duelist's favor (Note: this increases the chance of successful parry to 70% and a chance of a riposte attack opportunity at 26%; 48% with Improved Parry). Likewise, taking the Skill Focus (Parry) feat will add another 3 points.
With Spellcasting: When casting a spell in parry mode a character will parry attacks, but cannot make a counterattack until the next round. Once a character has begun to parry, any attempt to cast a spell must wait until the next round, meaning that to cast a spell while parrying requires beginning the spell while not under attack; a character under attack will never actually begin casting a spell waiting in their que. Generally a character will want to change from parry mode to defensive casting mode to throw spells for both safety and convenience.
A bug occurs because of how the engine handles multiple attacks. It groups all of a character's attacks into three "flurries" made at the beginning, middle, and end of the round. If an attacker makes two attacks in the first "flurry", the parry check will not be made against the second attack. This translates to the parrier being able to parry three attacks per opponent, yet can still parry more attacks from other opponents. This issue has been neither fixed, nor the description changed to reflect a deliberate balancing effect, since its original existence in the first game.
This skill does not exist in DnD 3.5