Cursed Luck adds a negative drn to the bearer's Defence and Protection rolls. Why is it all lowercase? Because, fortunately for the bearer, only one six-sided die is used as the base for this effect; regular DRNs use two.

Cursed Luck can, in fact, bring a fellow below the traditional minimum result for rolling a DRN (or even a regular 2d6). It thus increases the chance of being critically hit more than you might think, since it adds two potential values (zero and 1) for getting critically hit. It cannot make the result negative, however; the minimum is zero.

Does the effect apply to anything else? Perhaps. The public information on this is two games out-of-date.

Fateweaving is the most common source of this thing; attackers of Fateweavers, if their attempts aren't stopped by Petrification, Awe, or Vine Shield shenanigans, have their Luck cursed if they fail a Magic Resistance check versus the indicated value.