These have been known to very rarely drop a fire skull or mystery box. The mission chain has been disabled since NI has had codex, there is NO codex for these, yet.
This powerful box is used to enter competition dungeons. Save these mystery boxes for official events to rack up extra bonus points.wiki
Left overs from when NEVERDIE was involved, these are the keys for the Skeleton Pirate Ships, there is some code that needs entered as soon as you get in, if it is the wrong code you wake up outside of the instance.
The locations for these levels of Mystery Box are on the original SI map.