Stay close, my friends, and I will heal your wounds.
- Mythen of the Wild
Tranquility is a druid group healing spell learned at level 74. It continually heals all party members while the spell is being channeled. As the spell has a long cooldown and the druid cannot move or cast any other spells while channeling it, Tranquility is highly situational in its use.
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.
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.
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.
Patch 6.0.2 (14-October-2014): Tranquility now heals every Party and Raid member within range every 2 seconds for 8 seconds. It no longer places a periodic effect on each target. The total amount of healing it generates in Raids should be approximately the same as before this change.
Patch 5.3.0 (21-May-2013): Tranquility now targets 12 raid members (up from 5) each time it heals when used in a 25-player instance. This change also applies to players using the Symbiosis version of Tranquility.
Patch 4.1.0 (26-Apr-2011): Druids now innately have 100% pushback protection from damage while channeling Tranquility.
Patch 3.3.3 (2010-03-23): The combat log tooltip for this ability will no longer spuriously claim a range of 100 yards.
Patch 3.3.0 (08-Dec-2009): The cooldown on this spell has been reduced to 8 minutes, down from 10 minutes.
Patch 2.3.0 (13-Nov-2007): Tranquility now gains additional benefit from spell damage and healing bonuses.
Patch 2.0.3 (09-Jan-2007): The "Tranquility" base amount healed per second has increased by approximately 273%. The radius of effect has increased from 20 yards 30 yards. Mana cost has increased by 40%. The Cooldown has increased from 5 minutes to 10 minutes. The Duration has decreased from 10 seconds to 8 seconds.
Patch 1.8.0 (10-Oct-2005): Cooldown reduced to 5 minutes. Mana cost reduced by 25%.