Critical hits are physical attacks that hit crucial spots, dealing a significant amount of additional damage.
Melee[]
The PC's melee critical hits deal double damage, except for kicks which have the same variable damage multiplier as missiles. Monsters' melee critical hits deal either double or triple damage. For the PC there is a base chance of approximately 5% that a hit becomes a critical hit. This chance can increase to around 15% if the PC has good luck. PCs with the find weakness skill, certain weapons and items, and certain class powers receive additional chances to crit. Non-item chances will be individually doubled if the PC is attacking with a murderous weapon. Slaying melee weapons always score criticals against the creature type they slay.
For the PC, a critical hit is signified by the expression "with full force", for instance "You hit the giant rat with full force and slay it". For monsters, the message simply changes to "The <creature> critically hits you."
Since the damage multiplier is applied before PV, critical hits tend to be especially helpful against monsters with high PV. However, as critical hits do not normally bypass PV, it is possible to receive the "You hit the <creature> with full force but do not manage to harm it." message. In such a case even double the amount of damage wasn't enough to get past the monster's PV. Consider switching to a different form of offense.
If a critical hit landed by a monster would bring the PC's HP below 0, a PC that isn't doomed might have their HP reduced to 1 instead. This effect only seems to come into play when the PC is above a certain HP threshold depending on the strength of the attack, and applies to both melee and ranged criticals.
Missiles[]
Missile critical hits behave somewhat differently. The base chance for a critical is lower (~3-10% depending on luck), and appropriate sources of increased critical hit chance, including find weakness, seem to be around half as effective on ranged attacks. The damage modifier is different from melee — missile criticals can deal double, triple, quadruple or even quintuple damage. It is not known whether the same is true for monsters; what is known is that high level monsters that use missiles (such as fire giant kings or titans) can deal great amounts of damage at a distance, possibly for this reason.
Slaying missiles guarantee critical hits against the proper monster types. They also add extra damage if used against appropriate monsters (e.g. you can roll normal criticals with arrows of jelly slaying, but if the monster was not a jelly it won't affect the modifier), changing the multiplier to from triple to sextuple (x6).[1] Slaying ammunition is thus even more destructive as a method to defeat certain enemies than melee weapons. For characters that are very skilled at using their missile weapon to begin with (especially Archers), the combination with slaying ammunition can result in one-hit kills of even the most powerful creatures.
Scoring a critical with missiles is signified by the term "exactly", as in "You exactly hit the <creature>". Monster missile critical hits yield the message "You are critically wounded." instead of "You are wounded." following "The <creature> shoots a <missile> at you!".
Other[]
Critical hits should not be confused with penetration effects, i.e. hits that bypass PV. Critical hits do not in themselves bypass PV. It is possible for monsters' attacks to both penetrate armor and score a critical hit.
The message "the <creature> targets a vital spot and hits you" also does not signify a critical hit; this means that the monster's attack bypassed the PC's PV whilst (s)he was wearing light body armor. The message "the <creature> punches through your armor" is generated for attacks bypassing heavier body armor.