Is there a specific way to trigger this quest line? I’ve been trying for it for weeks but no dice so far 
I take it you did it once, and now are trying to get it to trigger again?
Not quite. Ive yet to have it trigger at all, the only reason I know about it is from digging on what the ‘shadow missions’ dailies were and why i hadn’t encountered one yet.
That is weird. After the update for Shadows to be in the mode, I immediately got the mission. Since then, I haven’t seen it again. @REYNOSOFUA11 said he went through the mission twice, but I have no clue how he got it to reoccur.
I don’t know either but it popped up alongside a regular shadow mission and thought it was one too until I read the in game text.