Hellfire was a warlock ability learned at level 22 for those with the Demonology specialization. It was a PBAoE spell during which pulses of fire radiated out each second from the caster, causing damage to enemies within range and the caster as well. This spell was a channeling spell, so interruptions could shorten the duration and thus the damage output.
This is a great spell to use with an experienced healer, if you're not being focused on, and in large groups of enemies. Once started you deal out tremendous damage but also receive as much. Compared to other AOE, Hellfire is incredibly mana-efficient, not taking into account the mana needed to heal. A popular combo is warlock/priest since Power Word: Shield minimizes channeling time lost, and if combined with Holy Nova, mobs die fast and the warlock is never in danger of dying.
Warlocks are unable to resist the damage that Hellfire does to themselves unless they use the [Sacrifice] ability of the Voidwalker, then the shield will take the damage until the shield fails or the effect is interrupted by moving.
If the warlock dies from this spell's effect, they take no durability damage. This allows the Warlock to save some repair costs when faced with certain doom. In such a scenario, a warlock will typically use [Life Tap] to get low on health fast, then use Hellfire. This is commonly referred to as "Ghost Walking" and is a common way to get into Scholomance without possessing the Skeleton key.