The set is supposed to proc 2 effects when you use a bone tyrant ability while in combat. I have the set "backbarred" and proc it using necrotic potency. It works maybe 4 out of 5 times -- despite the cooldown being over and being in combat. I can't find a pattern, so all I can say os that it's terribly inconsistent and doesn't proc when you expect it to.

I was tempted to keep using it despite this bug because I like the purple glow, but eventually got fed up and swapped it out.