HELP -- Need location of Crippling slash Warrior Elite

I've been hunting around for an Nightfall Elite warrior Skill called Crippling slash , yet I haven't had any luck finding it's location or anyother player that has ethier. It's down as an unlockable skill in PVP so where is it to be capped in PVE????

Please help I'm sure someone out there has it!

I have also yet to find this skill. Possible that they forgot to add it?

Its been talked that this is Nightfall's Assassin's Promise

I think its a certainty they have forgotten it. Other than "Your All Alone!" this is the only elite skill I wanted to use in Nightfall