If you haven't had the pleasure yet, maul is game breakingly powerful by the time you reach level 8 or so. That is to say its tons of fun, but after a while you wish something survived long enough to test if the AI would ever even move if given the chance. Last night my hero (level 12, 32 attack, and the impulsive trait) killed the bog elemental in two attacks. Or rather the first attack pecked away 8-10 hps at a time for ~15 or so attacks and then he got to move again immediately and finished him off. I did the same with the ice guy and the desert guy.
Here's the bigger problem. If the maul ability was uber rare maybe this would be okay, however it is almost impossible not to get. All you have to do is not kill your butcher men spawns and wait for them to start spawning armies of level 2 or 3 guys. Within 2-3 fights you'l have at least one berserker axe, or the equivalent sword.
I love the idea of the skill, its just to easy to abuse and break the game. Perhaps make the penalty on each successive attack be 15% or maybe even 20%.