Since day 1 of D2 I've never found one single Antiope-D. NOT. A. SINGLE. ONE. Yesterday, I opened more then 75 Banshe's Engrams. I got EVERYTHING but not a single Antiope-D. Last days I opened hundreds and hundreds of them, stil nothing! I talked to other players and they told me that they got the same problem: they simply can't find the Antiope-D, they've never found one! It's like that some players can't find it, like the -blam!-ing thing is bugged for some reason or is locked. Wtf?
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It's a banshee drop, easily confirmable in 5 minutes on the destiny wiki, not a Shaxx drop, don't bother with him. Are you making sure to check the engram preview to make sure it's in there before you open them? That's not a guarantee, and stuff not in the preview does drop, but in my experience you definitely have a better chance if it's visible as an option when you preview the engram. I believe the preview weapons used to change every 30 minutes, not sure about now.