So I got the triumph and title yesterday, and I think I've figured out what's going on (maybe?) So every week that I've tried, I've noticed the boss fight has been a different instance, like a different way of completing the mission. Now the first time I tried I actually failed because my light was too low and I just couldn't do it, but here's the thing - it was the same instance as I got this week. I completed it every week after, and then upon completing this instance, I got my triumph. SO, my theory, is that to complete the triumph you need to complete every instance of harbinger, with hawkmoon equipped by all 3 members etc. It is just a theory, but that seems to be how it happened for me, maybe someone else can confirm this does or doesn't work?