Black Fire

In Infernium you will encounter some floating black and burning particles in your adventure making a strange noise. Don't worry; they are harmless.

Once you go through them you will listen to a explosion but everything will be okay. Every time you go through one of these barriers you will enter a new area and if you come back to the previous area, the enemies will be reset: Cloak enemies will be back at their original position and Light Foes and Exploders will respawn again.

The Back Fire in the purgatory
If you go down in the purgatory you will go down a spiral stairway. Down you will find a back fire... but you can access it from both sides. This is a tricky one. If you traverse it from one side nothing will happen but if you do so from the other side you will see some ice patches appear in the room. Go up and you will reach the Ice Lands. Going to the Ice Lands is not a bad idea if you are in the permadeath state. If you want to go back to the purgatory just traverse the Black Fire from the other side.