We know, the achievement is bugged lol. The bottom crystal gives you the "Unpurified Lingering Mordrem Aether" which doesn't clear when you interact with the top crystal. It's supposed to give you the "Unpurified Lingering Undead Aether"
This is bugged again, I've been having the same issue and the "support" team just thank me for reporting the bug. Like, pretty poor form for an achievement that pretty much just came out. Let's see how long it takes them to fix this one.