|Don't you wish you could reroll that save?|
I've running with a "Luck" ability in my OSR games for a while now, and I've found it to be an excellent addition to a PC's resources. It doesn't take away tension. Instead, I've found it adds to the tense moments and makes gameplay just a tad more exciting. Of course, like most houserules that I've introduced in the past, I tend to initially make them needlessly complicated, almost as a justification for the rule itself.
Now I'm going to simplify it after using it with 2 different groups and 3 different campaigns.
"Luck" in Swords & Wizardry (or really any OSR ruleset)
Every PC starts out with 5 points of Luck. Thieves add 1 point to that total as well as Halflings (so a Halfling Thief would get 7 points)
Luck may be used the following ways:
* 1 point of luck may be spent to add + 2 to any D20 roll after the roll is made. This can turn a miss into a hit or a failed save into a save.
* 1 point of luck may be spent to add + 10 to any % roll after the roll is made. This can turn a failed remove traps roll into success
* 2 points of luck allows the PC to reroll any die (or dice for a damage spell) - attack, save, damage, even HP gained from level advancement if they have 2 pts of luck left in their pool when they are awarded their new level. It can also be used to force an opponent to reroll an attack, save or damage roll. The reroll replaces the original roll, even if it it worse than the roll it is replacing.
Luck may only be used once per "situation" or encounter.
Luck resets to the PC's maximum amount at the beginning of each session, so there is no reason to hoard it (except for the possible reroll of HP gain due to leveling)
If there is a way to add to a PCs permanent Luck Score, I haven't thought of one yet. I've sure it would involve some major forces if it did.
You could introduce something like an altar or a fountain, that if used properly, could restore some luck mid session. Hmmm, I should actually try that ;)