Unforgiving

 is a passive skill in Diablo III, unlocked at level 55.

In-game
The generation bonus applies all the time, both in and out of the combat.

Using this skill is the most trivial way to get the achievement for fighting with maximum Fury for 5 minutes. However, the credit will only be awarded when players bring the Fury below maximum level after 5 minutes of keeping it maxed out.