ToME Wiki
General Info
Skill in Critical-hits increases your chance of scoring a critical hit. If it does hit, it also figures the damage as if you had a much heavier weapon wielded than you do. Critical-hits skill also increases your Sword-mastery skill.
Code-related Information
First, you have to be wielding a weapon that weighs less than 5 pounds for this skill to trigger.
If this is the case, what the skill really does is increase the probability of scoring a critical hit, and then modifies the damage as if you were wielding a lot heavier weapon. Please understand there is already a probability you will score a critical hit, this skill just increases that probability. Specifically it increases the probability by skill*40/5000. What this means is at skill level 50, you will have a 2/5ths more likely chance to score a critical hit. At skill level 20 it would be 4/25. If the hit is critical, (if I understand it correctly), the damage is computed as if you were wielding a (skill times .8) heavier weapon. So at skill level 50, wielding a shadow blade, you would hit a critical hit 2/5ths more of the time than naturally, and if it were to hit, it would do damage as if the weight were 40 (50*.8) + 4.5, or 44.5 lb weapon.
Spending one skill point on your Critical-hits skill also increases your Sword-mastery skill by 0.05 skill points.
Editorial
In my opinion, MrConceit, and in others I have seen, pumping up this skill seems unwise. If you wanted to play with critical hits, it would seem more effective (and cost effective) to have Tulkas as your god. At prayer level 20, his spell Divine Aim allows every single hit of yours to be critical, regardless of weight. I presume the spell doesn't recalculate your weight to be higher, but I'd think that EVERY hit being critical would outweigh that. Plus 20 in prayer is a lot easier to get, and you don't have to have Sword-mastery.