Has a 1 in 26 chance of appearing. Will be buried into the rock with lightly glowing spikes around the surface. Your achievement progress will likely bug out at some point, and you will likely have to collect more than 5. Sometimes the counter resets when GSG release an update.
More details can be found here:
https://deeprockgalactic.gamepedia.com/Error_CubeDecember 2020: You can also find these in presents during the winter event. You can get multiple in every match, so this is the easiest way to complete this. Obviously this is time limited.
This achievement cannot be influenced or gamed in any way, it's purely random. It could take you 5 hours or 500 depending on luck.