Luck

When a creature gets lucky, they gain various combat advantages, at the same time increase the survival rate by lessening the incoming damages of certain types.

Effects
Every +/- point of luck gives the effected creature one point bonus/penalty to the following rolls Luck acts as a modifier after the above rolls have been made, but will never exceed maximum or drop below minimum value, nor will it affect the critical miss/hit rate, an attack roll of 1 is destined to miss and a 20 or 19 (depending on weapon proficiencies) guarantees hits
 * Outgoing physical attack rolls
 * Outgoing physical damage rolls
 * Healing rolls for spells such as Mass Cure and Mass Raise Dead
 * Incoming spell damage rolls
 * Incoming on-hit effect damage rolls

For multiple dice rolled, luck will apply the number of times of the dice thrown. Eg, in 2d4 roll luck will apply twice.

Luck types and Chant
There’re two types of luck status: The level 2 Luck spell implements Luck (non-cumulative) (133), as well as improves saving throws and thieving skills, which are not part of luck effects.
 * Luck bonus (22), it stacks with itself, also stacks with Luck (non-cumulative) (133)
 * Luck (non-cumulative) (133) does not stack with itself, but stacks with Luck bonus (22)

Chant spell grants the surrounding party members within 16 ft raidus Chant (non-cumulative) (131), its effect is the same as luck, the spell also tries to inflict Bad chant (non-cumulative) (137), just as negative point of luck, on enemies around 16 ft radius at the time of its activation. The two chant types do not stack with self, though they do stack with other effect types. Chant spell also modifies saving throws, which is not part of chant effects.

Causes

 * Story Mode (EE):  +6 luck bonus to party
 * Fatigue: -1 luck for every point of fatigue from 7, up to -94 luck at 100 fatigue
 * Intoxication: from -2 luck and +2 morale to -12 luck and +12 morale
 * Bard Song of unkitted bards (EE version 2.0+): +1 luck at level 1, +2 at level 15, +3 at level 20
 * Bard Song of blades (EE version 2.0+): +1 luck
 * Luck spell: +1 luck, -1 bonus to saves, +5 thieving skills
 * Wish spell possible effect: -5 luck for 60 seconds, everyone within 16 ft radius
 * Rabbit's Foot (Alora): +2 luck, -2 bonus to AC, +10 thief skills

Effects
Luck in IWD2 additionally affects critical miss/hit chances, which is especially useful because hammers, halberds, spears, axes and arrows deal triple damage on critical hits rather than double damage in IWD1 and BG, and there’s nothing in IWD2 that is immune to critical hits.

Causes

 * Fatigue: -1 luck for every point of fatigue from 7, up to -94 luck at 100 fatigue
 * Luck spell (IWD1): +1 luck, +1 saves, +5% thief skills, doesn't stack with the Lucky Scimitar
 * Luck spell (IWD2): +1 luck
 * Lucky Scimitar (IWD1): +1 luck, doesn't stack with Luck spell
 * Tymora's Melody (IWD1): +1 luck, +3 to saving throws, +5% thief skills
 * Tymora's Melody (IWD2): +1 luck, +3 to saving throws, +2 to thief skills, Alchemy, and Arcana Knowledge
 * Tymora's Loop (IWD2): +3 luck (extremely rare random drop)
 * Rabbit's Foot (modded IWD2): +1 luck
 * Breaking the mirror in the Ice Temple (IWD2): -20 luck

Sources & references

 * semiticgod’s posts on Luck: What it is and how it works thread