On Heroic (and higher) difficulty, the Wild Pyromaniacs and Unstable Voidfiends have slightly upgraded abilities:
If a Wild Pyromaniac's Fel Orb is not prevented or intercepted, it will eventually fall to the ground, leaving a patch of Chaotic Felblaze
If any players remain within the Volatile Voidstep as the Unstable Voidfiend teleports there, a Creeping Void patch will be created
These changes make it very important to kill these adds quickly and to manage their mechanics properly. Additional Chaotic Felblaze and Creeping Void patches produced by failing to execute these mechanics are likely to cause unexpected Shadowfel Annihilation explosions, which may wipe the raid.