You have to kill them after he obtains the sword. If you leave all 4 alive you don't do the achievement, which is the gold key.
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Also, I think it needs to be exactly 4. I believe I have been in groups that had 6 wraiths up and that does not count. Finally, he needs to literally take up sword. You cannot ult bomb everything as soon as he goes to center at 35%. There is some dialogue and animations before he takes up sword. You cannot kill anything before that completes.
Gotto keep all the ghosts after a certain point and wait till he picks up that sword. I dont think the number of ghost still up really matters. I vaguely remember being a tank I had more than four on me when the boss went for the sword, and we had the key.
Might be a glitch, but it's definitly not a constant bug.
Big fat Nord Dragon knight with a huge hammer. - Tank
Stealthy argonian witch templar - Healer (lowbie)
You have to be entirely sure that the wraiths are only killed after the boss taking the sword animation is done (not before or during the animation). This can easily be missed because everyone will be hyped or excited that you guys got 4 wraiths when the boss goes to the middle (which prematurely make the other members do ApE and possibly kill one or more wraiths before the boss is done taking the sword). Make sure the animation completes and trust me it is an easy thing to miss.
I don't know if this is common knowledge but you can get v16 monster shoulders from silver chest too. I got one yesterday and I didn't know that was possible.