Luck is not the chaotic ocean of
random chance most people believe it to be. If
you fail on a task (including an attack roll or a
defense roll), you can change the die result to
a natural 20. That still might not be enough to
succeed if the difficulty is higher than 6. Once
you use this ability, it is not available again until
after you make a ten-hour recovery roll. (Thief’s
Luck doesn’t work if you roll a natural 1 for an
attempted task, unless you also have and use the
ability Wrest From Chance.)