Prayer of Mending

Prayer of Mending is a reactive healing buff similar to Earth Shield. It will not actually heal the target when cast on them, but instead leave a buff on the target. When the buffed target takes any damage, the buff will heal the target, and move to another random raid member within 20 yards. The buff has a maximum of 5 charges (6 with 2-piece bonus from the tier 7 set), and will not proc from environmental damage (falling, lava, fatigue, etc) nor spells that reduce the caster's own HP such as Warlock Lifetap. (Spells that actually cause damage to the caster, such as Warlock Hellfire, or Priest Shadow Word: Death will still proc the buff.).

Prayer of Mending has a 10-second cooldown, added in patch 2.0.10, largely added to limit its effectiveness in PvP combat, for example to avoid instant self-heals without a cooldown.

The threat generated by ProM was changed to be given to the casting priest with patch 3.0.2. This was a side effect of changing the way ProM healing was registered so that it counted as the casting priest's healing.

Strategy
Prayer of Mending is often cast on the main tank before engaging combat, as it will begin the spell's 10-second cooldown and possibly allow mana regeneration before the fight begins. Waiting several seconds between casting the first Prayer of Mending(PrOM) and engaging combat allows a subsequent PrOM to be cast immediately (or almost immediately) after the initial spell procs.

Prayer of Mending can potentially save a party or raid in situations where damage is being taken throughout the group. Prayer of Mending's benefits over other multiple-target heal spells include large range and low mana cost (particularly in situations where most or all of the charges proc). There are some drawbacks, however, including the long cooldown, inability to stack on multiple targets, inability to heal multiple targets simultaneously, inability to target the heal after the first proc, and a maximum of 5 procs every 10 seconds. That said, combining PrOM and other group healing spells (e.g. Prayer of Healing, Circle of Healing, Tranquility) can be an extremely useful way to do a great deal of healing on a number of targets in a short period of time.

Although PrOM does not proc off of spells that reduce the caster's HP such as Life Tap, the damage blowback effect of Shadow Word: Death (when cast on a target it does not kill) will proc PrOM. Be sure to cast Shadow Word Death(SW:D) on a target that will not be killed (and remember that SW:D can crit) if you want to proc a PrOM buff you have. Also remember that you will be taking on a fairly large amount of threat with this tactic, both from the damage caused by SW:D and from the healing done by the PrOM.

Prayer of Mending also lends itself to group healing during mob grinds, particularly where the priest himself is capable of (albeit slowly) taking on a single mob at a time. Have each member of the party hold the aggro of separate mobs. As each person is hit, the PrOM will bounce from one to the other doing the sustained healing; allowing the priest to take a break from healbotting. Whenever a damage dealer in the party finishes their mob, they should target a new mob instead of coming back to assist the priest. The goal is to spread the damage, and thus the chance for PrOM to jump.

The 4-piece bonus of Hallowed Raiment increases healing done by this spell by 100 per proc and the 2-piece bonus of the Heroes' Regalia of Faith (and Valorous) increases the number of charges that ProM has to a total of 6.