Tranquility is a druid group healing spell that continually heals all party members while the spell is being channeled. As the spell has a long cooldown time and the druid cannot move or cast any other spells while channeling it, Tranquility is highly situational in its use.
Unlike most other heal over time effects, Tranquility ticks every two seconds. Each target is healed for the same amount, and the number of group members being healed will not affect the amount of healing done on each target.
Mechanically, Tranquility is sometimes considered to be a heal over time spell and other times not. Talents that affect "healing over time spells" sometimes do and sometimes do not affect Tranquility. For example, the spell is affected by the [Empowered Rejuvenation] talent, but not by the Tree of Life mana cost reduction.
[Genesis] increases the amount of healing done by 5%. [Gift of Nature] increases the healing done by a further 10%.
[Nature's Focus] reduces the pushback suffered from damage while casting Tranquility by 70%. Note that this only affects pushback from taking damage, and offers no protection against interrupts such as [Kick] or [Counterspell].
[Improved Tranquility] reduces threat caused by Tranquility by 100% and reduces the cooldown by 60%. This will reduce the cooldown down to 3.2 minutes.
[Empowered Rejuvenation] increases the benefit the spell gains from spell power by 20%. Note that this is a 20% increase to the amount gained; it does not mean that Tranquility gains an additional 20% of your spell power.
Tranquility is very mana efficient when many party members need healing, but is very ineffective when only one or two players need to be healed. Use it only when most of the group needs to be healed at the same time, such as when a boss has damaged everyone around him.
By spending two talent points into [Improved Tranquility], you can completely remove the threat generated by this spell. This not only allows you to provide a massive amount of healing, but lets you do so without having to worry about threat. If adds have come into the fight and the tank needs an extra 8 seconds to regain control of the situation, this talent may save your group.
If used during combat, it's advisable to cast [Barkskin] beforehand, so the spell does not lose duration to damage.
As this spell produces a large amount of healing in a short time, it can cause a lot of threat. Warning the rest of your party beforehand can save your life, as the tank can prepare to force the aggro back onto themselves with taunts after the spell has started.
Barkskin will not protect you from [Kick] or [Counterspell]. These types of interrupts not only make you waste your Tranquility cooldown, but also disable the spell school, leaving a Restoration druid mostly helpless. Given that your druid will be waving their hands in the air, causing the air to glitter, and producing a distinct noise, it is unlikely to pass unnoticed in PvP.
The "group" affected by this spell while in a raid group is the player's party, within a 30 yard range of the druid.
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Patch 3.3.0 (2009-12-08): The cooldown on this spell has been reduced to 8 minutes, down from 10 minutes. 
