Berserkers focus

Berserkers focus is a skill that adds endurance and nat resist, scaling off the user's willpower. Here is an example with ~370 willpower:

    berserkers focus            - endurance +6150 (15:54)
                                - nat resist +46 (15:54)

Using this skill incurs 6000 ms of lag.

From the in-game help file:

Usage: berserkers focus

When willpower is developed to a certain extent it is possible for the user to 
seek shelter within their own forceful, mental protection. Berserkers focus 
attempts to improve the user's natural resistance by using their own willpower 
to simply focus on "mind over matter." According to legend, the great Ishtl 
could focus with such dedication that he could fight armies of men at once, 
and leave the battlefield unscathed. In addition, this focus on the perfection
of the physical through mental means grants the user an increased level of
stamina.