"Musou" is your special attack. When the Musou bar is full, you may perform Evolution attacks at the end of your combos by chaining normal attacks, but only if your Combo is fully upgraded. In addition, by pressing L (using keyboard) or Circle (Default PS2 gamepad setting) you will perform a devastating special attack until your Musou gauge is empty or you release the key/button.
The Musou bar is filled by taking damage, dealing damage, and by picking up special items. In addition, while low on health, your Musou bar will slowly fill and the attack itself will be considered "Full Musou Mode" and will gain an increase in damage output.