Talk:Nightmare Mode (SAS4)/@comment-26317325-20151219110528/@comment-27374562-20151222014325

Nightmarish Necrosis Elites aren't required to get Cores or Keys; you can get those from any kind of Savage Necrosis Elite (it doesn't have to be Nightmarish, specifically, just Elite), but you DO have a better chance of getting them if it's Nightmarish.

And I'm not sure if your Level at opening matters, as much as your Level when you got the Box. I tried spamming the BLACK Boxes by opening them, then clearing out the app's cache through Settings and going back and reloading from the Cloud and opening them again to see if they would give me different items (on Android; this works with Live Event Rewards). They don't; so I think either the item is set when the BLACK Box is dropped, or it may just upload to the Cloud when you open them (it's probably the former).

What you could try if you're on Android is Force Upload; open the Box and see what you get; then reload from the Cloud to get the Box back; then, wait til you're at a higher Level, and open the box again to see if you get something different.