About negative temperature:
I understand how you can get it in a system and so on, but why does it not break the second law of thermodynamics, as entropy is decreasing in the system? Obviously I did not fully understand the second law, but what is the explanation?
It's a sliding scale thing; the universe's most cold temperature is absolute zero. In worlds that allow for non-entropic energy system, cold magic can dip down into the negative millions. It's often a very good way to put a permanent end to something.