Accuracy in Warcraft III (without having negative buff of Curse-based ability) is totally, 100%.
You can alter this fact, into adding Curse ability to the unit.
Well, I guess you will have to create the ability to 100 levels or so (1% for 1 level) or you can do 20 levels (5% for 1 level), your choice.
At start, the unit has 20% (example values) chance to miss on normal attack, over time, it will improve based on levels, items, etc, and your Curse will start to decrease to like 11% (example values) miss or so, this mean that your Accuracy has risen
A good idea to make Accuracy-based system is your unit should be based on Agility or whatever, like 7 Agility for -1% chance to miss upon attack. (this is just an idea)
Important element is the Curse-based ability gets into action.