Magical Resistance Rating

Overview
Magical Resistance Rating (abbreviated MRR) is a form of damage mitigation was that introduced with Update 23.

It provides a percentage reduction in damage from acid, sonic, fire, electric, cold, alignment, light, positive, negative, poison, and rust. MRR is granted through multiple sources (items, feats, enhancements) and stacks with itself. Multiple effects granting an Enhancement bonus to Magical Resistance do not stack. Character sheets show the Magical Resistance Rating of the player next to the elemental resistances.

MRR is applied after resistances. MRR is applied to the damage that passes through the resistance and this is always rounded down. As such if a player would take 20 damage before resists or MRR was applied and had 10 resistance and 60 MRR (62.5% reduction), the result is:
 * (20 - 10) * 37.5% = 3.75 round down to 3
 * 3 damage will be resisted.

Formula
100/(100+Rating)

Example: A character is hit for 100 points of damage.
 * 0 MRR takes full damage, 100 points.
 * 20 MRR takes 16.67% less damage, 83 points.
 * 100 MRR takes 50% less damage, 50 points.

MRR Cap
Different armor types can apply a cap for Magical Resistance Rating (U23):
 * Robes, Outfit, or empty armor slot: 50
 * Light Armor: 100
 * Medium Armor: No cap
 * Heavy Armor: No cap

Heavy and tower shields also prevent you from using Evasion.

Feats

 * Heavy Armor Training +3
 * Heavy Armor Combatant +6
 * Heavy Armor Master +9
 * Heavy Armor Champion +12

Items

 * Magical Resistance Rating offered by armor: None! (bonuses removed in Update 28 Patch 1)
 * Magical Resistance Rating offered by shield type (proficiency not needed):
 * Bucklers/Orbs: 0
 * Small shields: 5
 * Large shields: 10
 * Tower shields: 15
 * Enhancement bonus: Sheltering items (except Insightful Sheltering items)
 * Insight bonus: Magical Resistance items and Insightful Sheltering items
 * Mythic bonus: Mythic Shield Boost items

In addition, characters who are proficient with shields can also use the larger shields for defense against magical attacks that would normally require a Reflex saving throw. Instead of (or in addition to) using your Reflex saving throw to mitigate the damage you can deflect the damage off your shield. This is represented by increasing your Physical and Magical Resist Ratings against those types of attacks when using a shield. Note that you gain this additional mitigation whether you make your Reflex saving throw or not.

Physical and Magical Resist Rating multiplier against magical attacks that normally allow a Reflex saving throw.
 * Buckler: 1.0 (no additional mitigation)
 * Light Shield: 1.0 (no additional mitigation)
 * Heavy Shield: 2.0 (no evasion)
 * Tower Shield: 2.0 (no evasion)

Classes
Warlock
 * Entropic Ward +5 MRR. Warlock 6, Great Old One pact.
 * Paragon's Aegis +5 MRR. Warlocks receive it 5 times, at levels 12, 14, 16, 18, and 20, for a total of +25 MRR.

Feats

 * Past Life: Warlock: +3 MRR, can be stacked 3 times
 * Past Life: Deep Gnome: +3 MRR, can be stacked 3 times

Enhancements
Deep Gnome enhancements
 * Cores: 2+2+2

Enlightened Spirit enhancements
 * Shape Vestments: +5 in Light Armor, +10 in Medium Armor
 * Resilience of Soul: up to +6
 * Spiritual Ward: up to +13
 * Fortify Summons: up to +20 for summons / hirelings

Occult Slayer enhancements
 * Cores: +5/+5/+10 at levels 12/18/20
 * Parrying Bond: +1/2/3 while Weapon Bond 10+
 * Guarding Bond: +4/8/12 while Weapon Bond 30+/60+/90+
 * One Spirit: +4/8/12

Sacred Defender enhancements
 * Sacred Stance: up to +25
 * Harbored By Light: up to +25
 * Glorious Stand +5
 * Eternal Defender +10

Stalwart Defender enhancements
 * Stalwart Stance: up to +25

Tempest enhancements
 * Cores: 2+1+1+1+1+1
 * Dervish: +10
 * Improved Defense: +3