Critical hit

A critical hit (or crit for short) is an attack that strikes a vital organ and as a result, may do additional damage.

How It Works
A critical hit occurs when an attack roll results in a 20, before any bonuses are applied (such as from enchantments or proficiencies). A critical hit will always succeed in striking a target, regardless of the attacking creature's THAC0 or the target creature's armor class. Critical hits normally double the damage that would otherwise be inflicted, including any bonuses. Backstabs can also score critical hits, in which case the total backstab damage (multiplier and all) will be doubled. Some items (typically helmets) have the ability to protect against critical hits. This protection prevents the added damage; normal damage will still be done. [Creatures can be protected against critical hits without the use of special gear.]

Certain conditions can improve the chances of getting a critical hit:
 * Being proficient (1 point) or better in either the two-handed or single weapon styles will allow critical hits on a roll of either 19 or 20.
 * The Critical Strike and Smite HLAs allow all attacks made within a round to count as critical hits.
 * Some weapons (such as Rhyte's Last Arrow) offer increased chances of landing a critical hit.

Feedback

 * The combat log will provide feedback when a critical hit occurs (e.g., Minsc: Critical Hit)
 * A Party member will remark when they score a critical hit
 * If the target of a critical hit is protected from the additional damage, the combat log will report, "Critical Hit Averted"