Rain of Fire is a warlock ability learned at level 21 for those with the Destruction specialization. It is an AoE spell that deals fire damage; those with Destruction deal additional damage if the target is already under the effects of [Immolate].
Very situational; mobs can easily interrupt, it pulls threat, and has a higher mana cost. Mainly useful in instances for AoE.
Rain of Fire is an arcane spell used by conjurers, mages, sorcerers, wizards, and warlocks. Originally a spell created by the Conjurers of Azeroth, and other casters, it was later used by warlocks.
This is a very powerful spell that combines the natures of flame and water. It calls forth the essence of fire and delivers it through massive thunderheads of vapor and brimstone, creating an actual firestorm. This spell is very destructive, and will cause damage to any foes and buildings in its sphere of influence. Use this spell with care, for the rain of fire knows no friends.  Small meteorites rain from the sky, exploding on impact and burning all in their wake . Meteorites deal impact and fire damage to creatures, ignite and batter objects. 
Patch 5.4.0 (10-Sep-2013): Rain of Fire now has a reduced chance to generate Burning Embers.
Patch 5.1.0 (27-Nov-2012): Rain of Fire no longer causes a stun after hitting a target three times.; Rain of Fire no longer requires Immolate to be present to generate Burning Embers.; Rain of Fire's damage over time will now hit every 1 second, down from 2 seconds. The damage animation for the damage over time effect will no longer always display.
Patch 4.1.0 (26-Apr-2011): Rain of Fire damage has been increased by 25%.
Patch 4.0.6 (8-Feb-2011): Rain of Fire now remains at the same location it was cast at while on a moving transport.