Purges the enemy target, removing X beneficial magic effect(s).
It is as with Mannoroth's blood curse—a spiritual problem requires a spiritual solution.
Purge is a shaman ability that removes a "Magic" buff effect from one enemy target. Similar to a Priest's Dispel Magic but can not be used on allies. If the target has no buffs which can be purged, an error message will appear and the spell will not use any mana or trigger a global cooldown. The spell will cast on a Paladin with an Aura, but does not remove the Aura. Also the spell will cast if a target is immune through some mechanism such as Ice Block or Banish.
*Note: do not confuse the Purge spell from WC3/TFT shaman units; this spell does not slow the movement of the target nor inflict damage to summoned creatures.