Assuming you want it to be a seventh ability score, here’s an idea. It expands on the Action Point mechanic from Unearthed Arcana and Eberron Campaign Setting, which I have used often in my games and find quite useful.

The Luck Score

Luck is an ability score. It ranges from 0 upwards, and produces a Luck modifier equal to \$\lfloor Luck \div 2 \rfloor-5\$, as all ability scores do. Be sure to modify your character generation rules to account for a seventh ability score (e.g. multiply your Point Buy by 7/6, figure out a roughly median value for a new entry in your ability array, or just keep rolling).

To maintain the ability to improve ability scores as you level up, +1 to an ability score of choice is recommended at the following levels: 4, 7, 10, 14, 17, and 20, instead of 4, 8, 12, 16, and 20.

Luck Checks

Luck checks consist of rolling 1d20 and adding your Luck modifier. They may be used to gauge your results in a game of chance, or at other points when the DM decides luck is particularly relevant.

Skills

No skills are based purely on Luck; by definition if you can train in it, it’s not pure luck.

Luck 0

Having your luck reduced to 0 results in immediate and extremely unlikely death.

Luck Nonability

Creatures with a nonability for Luck, that is, Luck –, simply behave as if Luck did not exist as an ability score for them. Such creatures receive no Action Points (see below).

Creatures with and without Luck

I cannot go through all the creatures out there and assign them Luck scores, so here are some rules of thumb. Mindless creatures and non-living creatures (most constructs, the undead) usually have Luck –, as do most creatures of the Animal type and many of the Magical Beast type. Lawful outsiders also often lack a Luck score. For the rest, the more aggressive and reckless a creature is, the more likely it is to have a high Luck score.

Magical Bonuses to Luck

Bonuses that improve luck are rarer than those for other ability scores. In reliable, item form, they are almost unheard of, though powerful items like the luck blade may offer some. The rabbit’s luck spell is just like the cat’s grace spell but for Luck, and in that it is a 4th-level spell for clerics, druids, paladins, rangers, sorcerers, and wizards. Bards receive it as a 2nd-level spell, however. No mass rabbit’s luck spell is known.

Luck and Action Points

Action Points are the primary advantage of a high luck score, and the primary disadvantage of a low luck score. They are a creature’s ability to “get lucky” on things it does.

Action Points per Level

Now then, Luck controls a creature’s use of Action Points, which behave in mostly the same ways as the Action Points variant in Unearthed Arcana (this is the same variant used in Eberron Campaign Setting).

To wit, a creature gains a number of Action Points when it levels up equal to \$5 + \lfloor Level/2 \rfloor\$, so 5 at first level, 6 at second and third levels, 7 at fourth and fifth levels, and so on. These points, when spent, are not refreshed until the next time that creature levels up. Any points remaining when the creature levels up are lost; they do not “carry over.”

Good Luck

When you have Luck greater than 10 (“good luck”), the effect of expending an Action Point is to roll \$X\text{d}6\$, where \$X\$ is your Luck modifier, and taking the highest value rolled and adding it to the result of an attack roll, a skill check, an ability check, or a saving throw. You may only choose to use an Action Point after the numerical value of the roll is known, but before the effects of that roll are known.

Action Points may also be used to improve a number of feats and class features. See the Unearthed Arcana entry for more details.

Bad Luck

When a creature has Luck less than 10 (“bad luck”), a number of its Action Points are given to the DM to use against it. The number is equal to how much less than 10 the Luck score is (so 1 for Luck 9, 5 for Luck 5).

For these Action Points, the DM may choose to expend an Action Point and subtract the highest result of the \$X\text{d}6\$ rolled from an attack roll, a skill check, an ability check, or a saving throw. The DM must choose to do so before the unlucky creature has rolled, however. After the DM has done this, the Action Point is transferred back to the unlucky creature. The DM cannot use Action Points as some kind of “inverse” to one of the special uses for action points.

The Action Points not given to the DM cannot be used against the creature. However, these Action Points, as well as those the creature receives after the DM has used one of the Action Points he received, are not useful to the creature for the usual purpose of adding a bonus to an attack roll, a skill check, an ability check, or a saving throw. They may be useful for feats and class features, however.

Neutral Luck

For a creature with Luck 10, expending an Action Point in this manner does nothing (since zero d6s are rolled). It may use them for various special uses.

Frequency Limits

In no case may more than one Action Point be used on the same roll. The DM is furthermore prevented from expending a bad luck Action Point more than once per round for a given creature with bad luck.

Special Uses

Unearthed Arcana defines several special uses for action points to augment different maneuvers, feats, and class features. Furthermore, Eberron Campaign Setting and other books for that setting define feats and class features. These special uses and feats may be used with Luck-based action points. Some of them have modifications as indicated below:

• Boost Defense, Combat Expertise, Dodge: In each case, add your Luck modifier to the relevant AC bonus rather than doubling it.

• Spell Boost, Spell Recall: To boost or recall, one must have a Luck score equal to 10 + the spell’s level.

• Improved Critical: Extend the weapon’s threat range by your Luck bonus (if any). This extension happens after applying Improved Critical (i.e. it is not doubled).

• Improved Initiative: Add your Luck bonus (if any) to your Initiative roll, instead of doubling the +4 bonus.

• Metamagic feats: You can only apply this to metamagic feats that would otherwise increase the spell’s level by less than your Luck bonus.

• Power Attack: Determine your damage bonus as if the attack penalty you were taking was worse by an amount equal to your Luck bonus (if any).

• Spell Focus: Add your Luck bonus (if any) to the saving throw DC, instead of doubling the feat bonus. You may only do this if you have a Luck score of at least 10 + the spell’s level.

• Spell Penetration: Add your Luck bonus (if any) to the cast level check, instead of doubling the feat bonus. You may only do this if you have a Luck score of at least 10 + the spell’s level.

• Action Boost (EBCS): Instead of \$X\text{d}6\$ becoming \$X\text{d}8\$ permanently, this feat gives you the optional ability to change \$X\text{d}6\$ into \$\left(X-1\right)\text{d}8\$. Additionally, you may take Action Boost again to get the option of changing \$X\text{d}6\$ into \$\left(X-2\right)\text{d}10\$, and a third time to get the option of \$\left(X-3\right)\text{d}12\$. These options cannot be used if you have bad luck.

• Action Healing (FoE): If you have Luck 15, you may use two of the options rather than only one. If you have Luck 19, you may use all three at once, every time you use an Action Point this way.

• Action Surge (EBCS): This requires Luck 19 as well as BAB +3.

• Heroic Channeling (FoE): This requires Luck 15 in addition to its usual requirements.

• Heroic Companion (MoE): Your animal companion, special mount, homunculus, or other companion creature uses your Luck modifier for determining its use of your Action Points with this feat.

• Heroic Spirit (EBCS): The 3 bonus Action Points you receive cannot be used against you, even if you have bad luck.

• All feats based on Dragonmarks and applying to Action Points (there are a lot of those) may treat a character with Luck 15 as having a Least Dragonmark, a character with Luck 19 as having a Lesser Dragonmark, a character with Luck 23 as having a Greater Dragonmark, and a character with Luck 27 as having a Siberys Dargonmark for the purposes of requirements and effects of the feat on uses of Action Points. Any effects of the feat that do not involve Action Points are unusable if you do not actually have the required Dragonmark. In no situation does Luck allow you to use the spell-like abilities of a Dragonmark you do not have.

Finally, a new feat is added:

• Warded Against Disaster: When you have bad luck, the DM may use Action Points against you no more than once per encounter.

Conclusion and Analysis

I have played with Unearthed Arcana Action Points many, many times, and find the system enjoyable. The addition of an actual Luck ability score here changes a few things about the system, but because you are rolling a number of dice and choosing the highest, there are diminishing returns on very-high luck (which is why I changed Action Boost as I did). The very-limited ability to get additional Action Points, and the slow rate of refresh, ensures that Action Points remain special and important.

I particularly like that there’s a whole feast of already-existing feats and class features that immediately tie into Luck, through the existing Action Point options.

The bad luck mechanics are the ones that are most tricky. I believe I have found a solid balance there: they are decidedly negative, but not so bad that it’s insane to allow yourself to have negative Luck.

I am definitely interested in feedback on how this works out in a game.