[duke_commando]

Luck means 50% of successful attacks are negated.
So if you have ethereal and luck, 75% of attack will automatically miss and 50% of the rest that successfully do damage will also miss.
So luck is pretty uber on high defense units since few attacks will successfully hit them anyway, and half of those that manage to do damage will be negated.
This is my understanding of it from the description anyways.