5 Piece Wrath is..... 20% chance after using an offensive ability requiring rage that your next offensive ability require 5 less rage to use.
This ability gives you a buff that lasts for 10 seconds or till you use your next offensive ability.
However, when you have this buff it doesn't properly function with Concussion Blow. What the buff does instead is it makes the ability cost 5 rage. The regular ability costs 15 rage. This is a 10 rage reduction.