Turns out those monsters, those exact monsters, more or less, are back for this Ice Storm event. They’re no longer Cube Monsters, they’re Ice Monsters, part of the Ice Legion, and a new set of challenges will have you hunting them down for cosmetic rewards. The Fortnite Ice Storm event launched its list of challenges today and there are 13 total, with 3 live right now. The Ice "Fiends" for this specific challenge are the small ones that don't do anything, so only killing those will count toward the 250 you need. I’m trying to figure out if the “Ice Legion” includes the Ice Shards themselves, but I don’t think so.

Source: Forbes January 19, 2019 20:03 UTC