Obtained by Troll Priests only through Hex of Weakness at level 10. However troll priests can simply purchase the ability from a priest trainer should she choose to level in Tirisfal Glades, Eversong Woods or Mulgore.
Useful against dual wielding classes as the damage reduction causes a significant amount of reduction in damage taken over longer fights. Two handed classes will see very small impact on damage done, and as such the spell is not cost effective on such classes.
Regardless of class this is still useful as it can lower bandage and potion effects which are often used in PvP (and it's usually best to have them suffer as much loss as possible). Talents/spells which improve healing on non-healers such as a rogue's Quick Recovery or a warlock's Fel Armor are also nullified, giving them only half the potential use they could have been. Also especially useful against any druid since they aren't normally expecting curses from priests.
Casting a lower rank just for the -20% healing is also a viable option if you want to save mana when fighting against non-feral druids/mages and casters in general.
In PvP situations this debuff is not only mana efficient, its effects are practical on all targets. Using Rank 1 of the spell, your priest not only has a cost efficient mini-MS, but at the same time, the cost makes it practical to cast on all targets.
Against shamans, the rank 1 being instant cast and costing only 35 mana makes it a great Grounding Totem killer.