Serenity

Serenity is a Defensive Monk skill.

In-game
When activated, it makes the Monk completely Immune to Damage, even to attacks that normally kill outright, as well as all other harmful effects, for a short time, that is. Visually, their Armor vanishes, and the skin transforms into shining metal. There is no limit on amount of damage that this skill can absorb. Note that the cooldown does not start until the effect wears off, normally allowing the ability to be used every 19 seconds, that is to say.

Runes

 * Peaceful Repose: when activated, Serenity also heals the Monk. The amount of Life restored scales with level, up to 93,874 to 120,685 at level 70. This amount is further increased by 40% of the Health Globe healing bonus (unlocked at level 23).
 * Unwelcome Disturbance: also unleashes a deadly blast every second while Serenity lasts, hitting enemies within 20 yards for 438% damage as Physical (unlocked at level 29).
 * Tranquility: allies within 45 yards are also protected for 3 seconds, gaining protective shields that absorb damage (scales with level, up to 120158 at level 70) and make them immune to control impairing effects like Slow and Frozen. Technically, this damage is redirected to the Monk. Amount absorbed is increased by 40% of the Monk's Health Globe healing bonus (unlocked at level 39).
 * Ascension: increases the duration to 4 seconds (unlocked at level 47).
 * Instant Karma: while Serenity is active, the Monk can move through enemies and Waller affix barriers unhindered (unlocked at level 54).

Development
Originally, the Monk was supposed to have a similar skill named Impenetrable Defense. In the final game version, it was replaced with Serenity and Inner Sanctuary.

Trivia

 * A possible inspiration for this skill are the legends of Shaolin monks, so pure of soul that their bodies are like iron. These legends are partially based on real ability of some trained martial artists to break metal bars with their bare hands (and other body parts).
 * Skill of the same name exists in World of Warcraft as well, being an optional talent choice for Monks.