It's ability available in defensive stance for the warrior, you need a shield equiped to use the ability. It reduces all damage done by mobs by 75% for 10 seconds, I always use it with challenging shout so I take the attention of every mob around me and they tickle me for 7 dmg each.
It's a hell of a ability when used properly in a group, I used it when we had to fight 4 elite ogres including some ugly ogre boss. Blood rage - beserker stance - intercept - defensive stance - demoralizing shout to attract the attention of the others - challenging shout when the priest heals to keep the aggro followed by shield wall. Dead ogres.