The Fire Fiend curse creates fire powers for the user. They can use fire at their own will and even create weapons. However, they must learn self-control. If they get angry enough then things around them can explode.
The Fire Fiend curse cannot be ridden of. Once one has been cursed they will forever be cursed. Igneus feeds of the Fire Fiend curse as well. The Fire Fiend curse causes extreme pain to the user all the time as it is always happening within their bodies. If the user gets so mad then they can catch on fire and destroy anything in their path.
Igneus is waiting for his escape, it is said if he can destroy someone so much with his curse then he will be freed through them.