In D&d, ThAC0 is the equation of you'r weapon expertise to the armor of the opponent. All players at level 1 start with thac0 of 20. So, to hit a solder with an armor class 0 you need a roll of 20 on a 20 sided dice. An armor of 1 you have to make a roll of 19 OR 20 .. So, less armor, better chance. But still, depends on the class you use, thac0 drop by a -1 when you gain a new level. Fighter has -1 thac0 at each level gained, Thief and Bard -1 at each 2 levels gain, priest -2 at each 3 levels and Mage -1 to every 4 levels. So, you know fighters drop very fast thaco since they are weapon champions and Wizards are better casting spells that fighting with weapons. All the +1 on a magical weapon are added to you'r thaco as well then strengh score and weapon specialisation. Read the manual for more info. You cannot have a thaco of 200, not in d&d. thaco of 20 is poor so imagine 200 ...
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Maybe it's because I hate drows more than I hate Humans - Jarlaxle to Entreri