This article discusses luck in the games. Most of the information here has been provided by @semiticgod.
The confusion
The concept of luck is never explained in the game or the manuals, and most of the information they provide about luck is either wrong or misleading. This mini article tries to clarify the confusion.
The source of confusion is a number of inconsistencies in the manuals and the games:
- Luck is the name of a spell, but it’s also the name of an effect (like the blinded effect or the confused effect). So far, so good. Intuitively, you would expect that the Luck spell would simply improve somebody’s luck. However, that’s not the case. The Luck spell does improve luck, but it also gives several other bonuses (to saving throws and thieving skills) which are unrelated to luck (the luck effect, that is).
- In the Icewind Dale series luck works differently than in the Baldur's Gate series.
- There are two luck effects, one which stacks and one which doesn’t. The luck obtained from the Luck spell, Chant, and the Lucky Scimitar in IWD does not stack. However, the luck obtained from other sources does. So…
What is luck?
Luck is an effect that affects a character's performance in combat. Positive luck means better performance, negative luck means worse performance.
What does it do?
As stated above, it does different things in different games:
BG series: BG, BG2 and SoD
Luck affects to-hit rolls, physical damage dealt, and spell damage taken.
A lucky character hits more often, deals more damage with weapons, and takes less magical damage. By magical damage I mean all effects outside a weapon's base damage. That includes damage from spells, wands, abilities, weapon elemental damage, etc.
Luck bonuses are added to each to-hit roll and physical damage roll, and subtracted from each magical damage roll. Luck penalties work the other way around. But remember, this is AD&D, it’s always more complicated. Just adding a bonus to your roll would be too “simple”. And who wants something simple when you can have exactly the same thing, but twice as complicated? Well, you, me, the rest of the world… but not the AD&D creators, for sure. So this is the whole story:
Each time a die is rolled and a luck modification applies, the modification is added or subtracted from the roll, but without exceeding the roll natural limits. What? You didn’t understand that? Don’t worry, neither did I the first 200 times. Let's see: The "natural limits" of a roll are its maximum and minimum values. For instance, the natural limits of a d20 roll are 1 and 20. A luck modification cannot make the roll go beyond these maximum and minimum values. An example might help:
Flint the warrior has a +2 luck bonus. He attacks twice with his long sword (1d6 damage). The first time he attacks he rolls a 15. Adding the luck bonus, his effective attack roll is 17 (hit). He rolls a 3 in his damage roll. The luck bonus means he actually inflicts 5 damage (we assume there aren’t other bonuses/penalties).
The second time he rolls a 19 in his attack roll. The luck bonus would make that a 21, but since 21 is outside the natural limits of a d20 roll (1-20), it actually makes it a 20 (hit). By the way, he does NOT get a critical hit (unless he already had a critical with 19, of course). Then he rolls a 6 in his damage roll. Again, the luck bonus would make that an 8, but since 8 is outside the natural limits of a d6 (1-6), the result stays at 6, i.e., he only inflicts 6 points of damage.
Now a 5th level mage casts a Fire Ball on our poor Flint (and he fails his save throw). That’s 5d6 points of damage. The rolls of those 5 dice are: 1, 2, 3, 4, 5. Without his luck bonus, Flint would take 1+2+3+4+5 = 15 points of damage. However, the luck bonus reduces the damage he takes. The 5 becomes a 3, the 4 becomes a 2, the 3 becomes a 1. But again, the luck effect cannot modify a roll beyond its natural limits (1-6 in this case), so the the 2 just becomes a 1 and the 1 remains unchanged. The result is that he takes 1+1+1+2+3 = 8 points of damage.
Luck also affects other specific situations. For example, when a mage protected by Mirror Image is attacked, his attacker can either hit the mage or one of the images. The chances of each outcome depend on the luck of the mage. The higher his luck, the higher the chances that his attacker hits one of the images, and vice versa.
IWD series (IWD and IWD2)
Apart from the effects listed for the BG series, in the IWD series luck also affects critical hit and critical miss chances (so in our previous example our warrior does make a critical when he rolls 19). That's especially useful because hammers, halberds, spears, arrows, and axes deal triple damage rather than double damage on critical hits.
How do you get lucky (or unlucky)?
This is a list of the sources of luck (positive or negative) in all the games (except Planescape:Torment, I don't have information about that one). Some of these sources only affect luck itself, others create other effects as well.
All games
- Fatigue: -1 luck when the character becomes fatigued. An extra -1 for every extra four hours without resting.
- Intoxication: Penalty to luck and bonus to morale. The penalty goes from -2 to -12 and the bonus from +2 to +12, both depending on the amount of drink taken and the constitution of the character. The negative effects of the penalty always outweigh the benefits of the bonus. Slow Poison and Heal cure it.
All games bar IWD2 (i.e.: BG1, BG2, SoD and IWD)
- Luck spell: +1 luck, +1 to saves, +5% thief skills.
- Chant: +1 luck, +1 to saves, +1 to healing rolls (Staff of Curing, Mass Cure spell, etc.)
BG series: BG, BG2 and SoD
- Bard Song (unkitted): +1 luck at level 1, +2 at level 15, +3 at level 20.
- Blade song: +1 luck (doesn't scale with level).
Baldur's Gate
- Rabbit's Foot (Alora): +2 luck, +2 AC, +10% thief skills.
Baldur's Gate II
- Wish spell: Two of its effects affect luck:
- One intoxicates enemies, giving them a -12 luck penalty for 200 rounds. It bypasses magic resistance and offers no saving throw. It is dispellable and can be blocked by Spell Immunity: Enchantment and possibly Minor Globe of Invulnerability, as it is a level 3 Enchantment spell. Slow Poison and Heal cure it.
- The other effect gives -5 luck for 60 seconds party-wide.
IWD series: IWD and IWD2
- Chaos of battle (Priest of Tempus ability): 10% chance of a luck bonus between +1 and +5, depending on level.
Icewind Dale
- Lucky Scimitar: +1 luck.
- Tymora's Melody: +1 luck, +3 to saving throws, +5% thief skills.
Icewind Dale 2
- Luck spell: +1 luck.
- Tymora's Melody: +1 luck, +3 to saving throws, +2 to thief skills, Alchemy, and Knowledge (Arcana).
- Tymora's Loop (extremely rare random drop): +3 luck.
- Young Ned's Knucky: +2 to Luck, +2 to saving throws.
- Breaking the mirror in the Ice Temple: -20 luck.
- Chant: Unknown.
Stacking luck effects
As stated above, there are two luck effects, one which stacks and one which doesn’t.
- Luck effect 1: Obtained from the Luck spell, Chant, and the Lucky Scimitar in IWD. It does not stack with itself, but it does stack with luck effect 2.
- Luck effect 2: Obtained from other sources. It stacks with itself and with luck effect 1.
Example:
- After a long adventure, a priest and a blade get fatigued. They get a -1 penalty to luck.
- The priest casts Chant. Chant gives them +1 bonus to luck. This is the luck effect 1, which stacks with the luck effect 2 from being tired. The combined effect is that their luck becomes neutral (1-1=0).
- The blade casts Luck on himself. Both the Chant spell and the Luck spell create the luck effect 1, which doesn't stack with itself, so he doesn't get a luck bonus.
- The blade starts to sing. He and the priest get a +1 bonus to luck because the blade song (+1 to luck) creates the luck effect 2, which stacks with the luck effect 1 from Chant.
Note: Originally this post was a set of questions about luck. I include the original post in the spoiler for reference:
I’m trying to understand the concept of luck in the game, but most of the information I’ve found is confusing or contradictory. I’ve compiled information from the manuals and the forums trying to clarify it:
Manuals
Survival Guide:
- While the [bard] song is playing, the party’s morale and luck are higher.
- A character can continue to operate at peak efficiency for 24 hours in game-time (2 hours realtime). After that, the characters will start to complain and their attributes begin to suffer. For every four hours beyond this 24-hour mark, the player will receive a –1 luck penalty (–1 to all rolls).
- Intoxicated characters gain a morale bonus, but receive a luck penalty.
MM&M:
- Luck spell: The recipient of this spell is lucky in everything they do for the next 3 rounds, receiving a 5% bonus to any actions. This includes Saving Throws, to-hit rolls, thieving skills, etc.
Adventurer’s guide:
- Fatigued characters suffer a –1 penalty to luck; every four hours after becoming fatigued, this penalty increases by one. […] Luck affects every d20 roll your character makes.
- THAC0 and Armor Class: The d20 roll can also be modified by circumstances such as spells, special abilities, and luck.
- Luck spell: This spell’s recipient is lucky in everything they do for 3 rounds. They receive a +1 bonus to all actions, including Saving Throws, to-hit rolls, thieving skills, and anything else requiring a d20 roll. Contradictory: Thieving skills don't use d20 rolls.
Summary:
- Stuff that affects luck:
- Bard song: Unspecified bonus.
- Intoxication: Unspecified penalty.
- Fatigue: -1 penalty for every four hours fatigued.
- Luck spell: 5% bonus to all actions (?). Alternatively, +1 bonus to all actions.
- Effects of luck:
- -1 to all rolls for each -1 penalty.
- Affects Saving Throws, to-hit rolls, thieving skills, etc.
- Affects d20 rolls.
Forum
These are summaries of the most interesting posts I've read. They're not quotes, so I hope I got them right:
- @lunar: Luck adjusts damage rolls. When casting offensive spells which involve dice rolling your luck bonus is added to each die roll, up to the maximum die roll result.
- @JuliusBorisov: A luck bonus reduces the damage taken from magic and elemental damage by one point per die, down to a minimum of 1.
- @Semiticgod: There are two different effects called luck, and they work in more or less different ways.
- @Ark_Tolei: Luck doesn’t affect damage (dealt or taken).
Conclusion
My completely uneducated guess is that the key to understanding this is
this post by semiticgod. If I get that right (and I probably don’t) the source of the confusion is the fact that the word “luck” is used in the game to describe two different effects rather than one. These two effects work similarly in some aspects and differently in others, hence the contradictions and the confusion.
What do you think?
. For example I might suggest that you must always have a full party where possible but no resurrections are allowed, someone else might recommend that potions can't be used in combat, and yet another person might suggest a 'carousing' rule - your characters like to party and blow 10% of their cash at an inn at every available opportunity in classic sword & sorcery fashion 
. After about 5 or 6 people make a suggestion things could get very interesting indeed. Any thoughts on what sorts of rules might be interesting for you?
. Anyone want to contribute something with a bit more punch (or sneakiness), or would people prefer to go with a full party of casters ...
