Warlock players in World of Warcraft may learn a skill called Inferno. The skill calls forth an infernal to the warlock's aid for one minute (Prior to Patch 3.0.2 the length of time for summoned Infernal was 5 minutes; after this time, the Infernal would become hostile and had to be re-enslaved.) The ability is available to Warlocks at level 50 after completing a quest line from Niby the Almighty in Felwood, where the warlock must defeat Kroshius to obtain his Infernal Core. Warlocks may also learn the ability from reading [Grimoire of Inferno], which drops off Scarshield Warlock in Blackrock Spire.
Infernals has no warlock controlled abilities, however they do have Immolation, a passive ability which deals AoE fire damage (40 + 100% of the infernal's spell power every 2 seconds).
Casting the spell consumes an Infernal Stone (purchasable at reagent vendors for 50 each) and creates an Infernal under initial control of the Warlock.
Summons a meteor from the Twisting Nether, causing 200 Fire damage and stunning all enemy targets in the area for 2 sec. An Infernal rises from the crater, under the command of the caster for 1 min.
The Infernal will remain under the warlock's command for one minute. It can tank multiple mobs better than a Voidwalker; aside from the quite respectable melee damage it does, it also has a fire aura that will damage all units within melee range that aren't outright immune to fire. However, Infernals do not have a taunt ability, so if you do pull aggro they are generally incapable of re-acquiring the attention of a mob. The Infernal is especially useful in battlegrounds, you can drop inferno on a group of players to stun them and deal some AOE damage followed by Shadowfury it can give up to 5 sec group stun which is more then enough time to deal massive AOE damage for your allies. Or simply summon it and let it loose many people overestimate the infernal and will back off. This may buy some time for your own faction — unless they have a warlock who knows what they are doing and simply banish it.
Patch 3.3.0 (08-Dec-2009): This pet now innately have Avoidance like all other warlock pets.