Inferno (warlock ability)

Inferno is an warlock AoE spell that summons an infernal and requires an. Once in the infernal is summoned, it will automatically be under the effects of Enslave Demon for one minute, after which it will die. Inferno has a cooldown period of ten minutes. An Inferno has a permanent Immolation aura that breaks CC. In Alterac Valley players tend to over estimate an Infernal's strength and can be used as a rather effective distraction target.

Inferno has a 100% spell power coefficient.

Source
This spell can be learned from or the quest. However it is much easier for high level warlocks to get this spell through the book rather than the quests. The book has a roughly 3% drop rate from Scarshield Warlocks in Lower Blackrock Spire, and should drop after 1 or 2 runs.

Cataclysm
Inferno will no longer replace your current minion. Instead, the Infernal will attack the target of Bane of Agony or Bane of Doom.