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 (37.5% 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.

Increasing MRR cap
Few rare effects exist that allow your character to increase the MRR cap restriction imposed by your armor.
 * Beacon of Magic item sets: +10 heroic / +20 legendary for three piece item set
 * Nystul's Mystical Defense: +40 for five piece filigree set

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

Past Lives
 * Past Life: Deep Gnome: +3, stacks 3 times
 * Past Life: Warlock: +3, stacks 3 times

Epic / Destiny
 * Construct Exemplar +10
 * Deific Warding +10 (up to +30)
 * Dreamscape +15 situationally

Legendary
 * Scion of Elysium +25 for summons / hirelings
 * Scion of Limbo +30 situationally
 * Scion of the Plane of Fire +10
 * Scion of the Plane of Water +20

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)(Stacks with the Sheltering enhancement):
 * Bucklers/Orbs: 0
 * Small shields: 5
 * Large shields: 10
 * Tower shields: 15
 * Enhancement bonus: see Sheltering items
 * Insight bonus
 * Quality bonus
 * Mythic bonus: Mythic Boost items (armor, shields, boots, cloaks, headgear / necklaces / bracers)
 * Luck bonus: Unbreakable Adamancy set bonus
 * Artifact bonus
 * Slave Lords Crafting set bonus
 * Ravenloft item sets


 * Reaper bonus
 * Profane bonus:

Shield multiplier
Characters proficient with shields can also use the larger shields for defense against magical attacks that would normally require a Reflex saving throw. 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.
 * You gain this additional mitigation whether you make your Reflex saving throw or not.
 * You don't have to be actively blocking to gain this benefit.

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.

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
 * Cores: +5/+5/+10 at levels 12/18/20
 * Sacred Stance: up to +25
 * Harbored By Light: up to +25

Stalwart Defender enhancements
 * Stalwart Stance: up to +25
 * Capstone: +10 passive and +50 active action boost

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