IMO:
Power Attack: when you would attack with this weapon you can forgo an attack. If you do, the next attack you make with this weapon before the end of your next turn deals double damage.
If you forgo 2 attacks, the damage is tripled, if you forgo 3 attacks it is quadrupled.
Another option would be they can only be used once a turn, like a sneak attack - possibly activated by using a bonus action (emulating an off-hand attack in a manner of speaking).