Damages creatures you have mastered slaying.

This ability when activated allows you to channel the spirits of your slain foes and strike out against beasts that you have mastered slaying.

This is a special ability you get as you complete the Slayer Skill achievements. It has a reuse time of 1 minute which can be reduced up to 15 seconds by completing the Slayer General achievements for Hastened Banestrike. You must be an All-Access (gold) member to do this achievement.

You can find some tips on where to find your foes at Banestrike Creatures. There is also a list of all the achievements for Banestrike and Hastened Banestrike at Slayer System.