You send a ghostly soul into the target, dealing X Shadow damage and increasing all damage done by your spells on the target by 25% for 8 sec.
Haunt is a warlock ability learned at level 62 for those with the Affliction specialization. It causes direct damage and places a debuff on the target, amplifying all damage done by the warlock's spells against it.
In PvE, this spell is to be kept up on a boss target at all times, significantly boosting the Affliction Warlock's overall single-target DPS.
Patch changes
Patch 4.1.0 (26-Apr-2011): Haunt damage has been increased by 30%.
Patch 3.1.0 (14-Apr-2009): Now only increases your shadow damage-over-time spells on the target.
Patch 3.0.8 (2009-01-20): Will now gain correct damage modifiers for the creature type of the target. In addition, this ability is now limited to a single target as intended. This also now works properly when refreshed on its current target.
Patch 3.0.3 (04-Nov-2008): The cooldown has been changed to 8 sec, and refreshing the Haunt will trigger the heal from the existing Haunt. This ability will no longer heal for incorrect amounts or fail to heal when the Haunt damage was the killing blow.
/Patch 3.0.2 (14-Oct-2008): Haunt will now heal for 100%.