This set seems interesting, even though it's been around a while. The five piece bonus applies Major Defile, just like the S&B skill Reverberating Bash.
"When you deal melee damage, you apply Major Defile to the enemy for 5 seconds, reducing their healing received by 30%. This effect can occur once every 5 seconds."
Here's what I'm wondering though. Does it affect multiple players simultaneously on a melee AoE attack? Or is it just one player every five seconds?