Insanity

From the helpfile:

Usage: insanity <target>
Usage: insanity (when already in battle, this ability will automatically target
      your opponent if no target is supplied)

Insanity is the true measure of a ruanbaere. This skill requires massive 
exertion on the part of the ruanbaere to work up the fever, and drains more 
endurance from the user with each attempted attack. In addition to the huge 
cost in endurance, the ruanbaere can do little else but ride out the fever of 
insanity. While the cost may seem great indeed, and dangerous, the sight of an 
insane ruanbaere will send even the most seasoned warrior into flight.

The benefits of insanity are three fold. Much like berserk, the ruanbaere most
powerful skill's most noticeable effect is the increase in damage it brings. 
The stronger the ruanbaere, the more damaging his blows will be. Another 
effect, for the older, more experienced ruanbaere, is an increase in his 
ability to land attacks while insane. The greater the power of his mind, and 
the more experience he has, the more the ruanbaere will be able to control his 
body while in the throws of insanity. Finally, the power behind an attack from 
a ruanbaere under the effects of insanity make it impossible to parry the 
blows.

When working himself into the fever of insanity, the ruanbaere's fortitude is 
compared to that of his target. The stauncher the target, the harder it is for 
the insanity to affect him. Some say this is because the ruanbaere actually 
makes his target somewhat insane when he attempts to enter the fever, and the 
more robust the target the harder it is to affect him.

Note that there is a defensive drawback to insanity - an insane barbarian is
easier to hit.

Mastery of this skill beyond 100% improves insanity in two important ways:
1) The damage benefit of insanity is improved with greater mastery.
2) With enough mastery, insanity's endurance drain can be reduced.