During the Aerlinthe Quest, 3 events will trigger Hellfires to spawn in the three main volcanos of Dereth - Aerlinthe, Crater (Mt. Espar), and Mt. Lethe.  The way this works is that when the Behemoth of Tenkardunn is active, the Hellfire near it will also become active.  Once defeated, a Hellfire will spawn at Mt. Espar.  After this one is killed, a final one will spawn at Mt. Lethe.  With each of these spawnings will be a large number of fire elementals of all shapes and sizes.  There will be quite a number of Hellfires among them.  Once the correct one is killed, the others in the area will die as well.

  Each Fire infusion may be used on the Sword of Lost Light to improve its characteristics.  They may also be turn in to Lily og Glendon Wood at 32N, 27.2E for an experience award (timer not yet known).  These rewards are shown in the below tables.  Each Hellfire drops three infusions though and there are only two awards that have been discovered (XP, SoLL upgrades).  Kind of makes you wonder...