Damage modifiers refer to the buffs or debuffs applied to a unit that causes it to deal or receive increased or decreased damage. These are percentage values. They don't add extra flat damage but modify all damage normally done. These buffs or debuffs are only received by champion abilities, neutral buffs, on-hit effects from items and runes.
Damage dealt modifier
These are buffs that allow the champion that has them to deal increased or decreased magic or physical damage from basic attacks, abilities, and item actives. The raw value from the attack or ability is increased or decreased by the modifier and then applied to the target.
All damage dealt modifiers stack multiplicatively. Since multiplication is commutative, the order is irrelevant. Example: If 
Swain uses 
Torment while affected by 
Sona's 
Aria of Perseverance, his damage to the afflicted target will be modified by 90% (120% × 75%).
Increasing damage dealt
Champion abilities
Note: Only the damage modifier effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
Alistar's Trample deals 100% increased damage to minions.- Ashe's Frost Shot causes her to deal 10% + (critical strike chance × (1 + bonus critical strike damage))% increased damage to targets affected by Frost.
- Aurelion Sol's Celestial Expansion causes his stars to deal 50% increased damage when active.
- Corki's Hextech Munitions changes his basic attacks to deal 50% physical damage and 50% magic.
- Hecarim's Rampage deals 50% increased damage to monsters and champions.
- Jinx's Switcheroo! (specifically Fishbones) modifies Jinx's autoattacks to deal 110% attack damage and to splash.
- Kha'Zix's Taste Their Fear deals 50% increased damage to isolated targets.
- Lucian's The Culling deals 400% increased damage to minions.
- Miss Fortune's Bullet Time can critically strike per wave, dealing 20% × (1 + bonus critical damage) bonus damage.
- Miss Fortune's Double Up deals 50% increased damage to the second target if Double Up kills the first target.
- Nidalee's Javelin Toss damage is by 「 1% per 3.875 」「 25.8% per 100 」 units traveled, capping at 200% bonus damage.
- Pantheon's Certain Death grants Spear Shot 100% additional damage to targets below 15% health.
- Pantheon's Grand Skyfall deals up to 100% increased damage to enemies hit depending on their proximity to the center.
- Pantheon's Heartseeker Strike deals 100% increased damage to champions.
- Rek'Sai's Furious Bite deals 1% increased damage per point of Fury, dealing true damage at full Fury.
- Renekton's Reign of Anger consumes 50 Fury to increase a basic ability's damage by 50%.
- Riven's Wind Slash deals 2.66% increased damage for each 1% health the targets are missing (maximizing at ~200% increased damage when targets are at 25% health or less).
- Rumble's Junkyard Titan: Having 50 or more heat causes Rumble's Flamespitter and Electro Harpoon to deal 50% increased damage.
- Ryze's Overload deals 60 / 70 / 80 / 90 / 100% bonus damage against targets marked by Spell Flux.
- Shyvana's Twin Bite causes her next autoattack to deal 40 / 55 / 70 / 85 / 100% bonus damage and trigger on-hit effects twice.
- Sion's Decimating Smash deals 33.3% increased damage against monsters and 66.7% against champions.
- Sion's Roar of the Slayer deals 30% increased damage to secondary targets.
- Sion's Unstoppable Onslaught deals up to 100% increased damage based on how long he charged.
- Sona's Hymn of Valor grants her Power Chord 50% additional damage.
- Swain's Torment afflicts his target, dealing damage to them over time and causing Swain to deal 20% increased damage to it for 4 seconds from all damage sources.
- Syndra's Force of Will deals 20% increase true damage when at maximum rank.
- Tristana's Explosive Charge deals 30% increased damage every time the target gets damaged by her autoattacks or abilities, up to 120% increased damage.
- Vayne's Tumble grants her next attack 30% / 35% / 40% / 45% / 50% increased damage.
- Vi's Vault Breaker deals 33.3% increased damage against champions.
- Volibear's Frenzy deals 1% increased damage for each 1% of health the target is missing.
- Xerath's Eye of Destruction deals 50% increased damage to targets in the center.
- Yasuo's Sweeping Blade casts increase the next dash's base damage by 25% for 5 seconds, up to 50% increased damage.
- Ziggs' Mega Inferno Bomb deals 25% bonus damage to targets in the center.
- Ziggs' Short Fuse deals 100% increased damage against structures.
Runes
Flat bonus damage
While technically not damage modifiers, these abilities are included for completeness.
General
- Basic attacks against lane minions deal 5 bonus damage.
Champion abilities
- Ashe's Ranger's Focus causes her autoattacks to deal 115 / 120 / 125 / 130 / 135% AD total physical damage over 5 shots.
- Gangplank's Powder Keg deals 60 / 90 / 120 / 150 / 180 bonus damage to champions.
- Lucian's Lightslinger deals 100% AD against minions on the second shot.
- Miss Fortune's Love Tap causes her to deal 60 - 100% AD bonus physical damage on auto attacks against unmarked targets.
- Skarner's Crystal Slash deals 33 / 36 / 39 / 42 / 45% AD (+ 30% AP) on subsequent casts for 4 seconds after hitting an enemy.
Items
- Doran's Shield: Unlike most sources of bonus damage, this is added after damage modifier calculations.
Decreasing damage dealt
Champion abilities
Note: Only the damage modifier effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
- Jhin's Whisper makes his critical strikes deal 25% less damage.
- Kalista's Martial Poise modifies her autoattacks to only deal 90% AD.
- Kled's Skaarl the Cowardly Lizard modifies his autoattacks to only deal 80% AD to champions when he is dismounted.
- Sona's Aria of Perseverance enhances her Power Chord, to debuff her target to deal 25% (+ 4% per 100 AP) less damage for 3 seconds.
- Yasuo's Way of the Wanderer makes his critical strikes deal only 90% damage, reduced to 75% on Steel Tempest.
Summoner Spell
- Exhaust: Weakens the target enemy champion for 3 seconds, slowing them by 30% and reducing their damage dealt by 35%. ( 650 range; 210 seconds cooldown)
Damage received modifier
These are buffs or debuffs that allow the champion that have them to receive increased or decreased magic or physical damage from autoattacks, abilities and item's actives. The final value from the attack or ability is increased or decreased by the modifier and then directly applied to the target's health.
Armor and magic resistance damage reduction percentage count as one source of damage received modifier against physical and magical damage respectively. For example, a champion with 300 armor has 75% damage reduction from physical damage.
Damage reduction from armor and magic resistance and from any other sources stack multiplicatively.
Increasing damage received
Note: Only the damage modifier effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
Champion abilities
- Vladimir's Hemoplague affects all targets hit to receive 10% increased damage from all sources for the next 5 seconds.
Items
Decreasing damage received
Champion abilities
Note: Only the damage modifier effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
- Alistar's Unbreakable Will grants him 55% / 65% / 75% damage reduction for 7 seconds.
- Annie's Molten Shield grants her 16% / 22% / 28% / 34% / 40% damage reduction for 3 seconds.
- Braum's Unbreakable grants him 30% / 32.5% / 35% / 37.5% / 40% damage reduction from oncoming sources (excluding true damage and towers) for 3 / 3.25 / 3.5 / 3.75 / 4 seconds. The damage reduction is increased to 100% for the first source of champion damage that would be reduced.
- Galio's Shield of Durand grants him 20 / 25 / 30 / 35 / 40% (+ 8% per 100 bonus MR) damage reduction while channeling for up to 2 seconds and for another 2 seconds after casting the ability again.
- Galio's Hero's Entrance grants the target ally 20 / 30 / 40%(+ 8% per 100 bonus MR) damage reduction until he lands.
- Garen's Courage places a defensive shield on himself, decreasing all damage taken by 30% for 2 / 3 / 4 / 5 / 6 seconds.
- Gragas' Drunken Rage makes him take a long swig from his barrel, disabling his ability to cast or attack for 1 second and receive 10% / 12% / 14% / 16% / 18% reduced damage for 3 seconds.
- Kassadin's Void Stone reduces all magic damage taken by 15%.
- Malzahar's Void Shift grants him 90% damage reduction against the first spell or autoattack that hits him, lingering for 0.25 seconds.
- Master Yi's Meditate grants him 50 / 55 / 60 / 65 / 70% damage reduction up to 4 seconds while channeling.
- Warwick's Primal Howl grants him 35 / 40 / 45 / 50 / 55% damage reduction for 2 seconds or until he reactivates the ability.
Summoner Spells
- Challenging Smite: Deals 900 true damage to the target medium / large monster, modified to 500 (+ 10% of target's maximum health) against enemy minions or pets. Using Smite against a medium / large monster heals for 80 (+ 7.5% of your maximum health). Challenging Smite can be cast on enemy champions to mark them for 4 seconds, reducing the target's damage dealt to you by 10%, and empowering your basic attacks and ability damage against them to apply a burn that deals them 48 − 125 (based on level) true damage over 3.25 seconds. ( 500 range; 15 seconds cooldown; 90 seconds recharge time; 2 charges)
Items
- Ninja Tabi: Passive: Unique: Reduces incoming damage from all basic attacks, excluding turret attacks, by 12%.
- Phantom Dancer:
Damage immunity
There are some instances in the game where a champion is immune to receiving any kind of damage (including true damage) for a few seconds.
Champions
- Aatrox's Blood Well grants him damage immunity while reviving and restoring health.
- Bard's Tempered Fate grants affected units damage immunity for 2.5 seconds.
- Fiora's Riposte grants her damage immunity for 0.75 seconds.
- Kalista's Fate's Call grants the oathsworn ally damage immunity for up to 4 seconds.
- Kayle's Intervention grants the target ally champion damage immunity for 2 / 2.5 / 3 seconds.
- Kindred's Lamb's Respite grants all units inside the affected area become immune to damage when they are reduced to 10% of their maximum health.
- Lissandra's Frozen Tomb grants her damage immunity for 2.5 seconds if cast on herself.
- Taric's Cosmic Radiance grants him and all nearby allied champions damage immunity for 2.5 seconds after a 2.5 second delay.
- Zilean's Chronoshift grants the target damage immunity for 2 seconds if it takes fatal damage within 5 seconds.
Items
- Guardian Angel: Passive: Unique – Rebirth: Upon taking lethal damage, enter resurrection for 4 seconds, during which you are invulnerable, untargetable, and unable to act, and afterwards heal for 50% of base health and restore 100% of maximum mana (300 (starts after resurrecting) second cooldown).
- Stopwatch: Active: Unique – Stasis: Put yourself in stasis for 2.5 seconds, rendering you untargetable and invulnerable for the duration but also unable to move, declare basic attacks, cast abilities, use summoner spells, or activate items.
- Wooglet's Witchcap: Active: Unique – Stasis: Put yourself in stasis for 2.5 seconds, rendering you untargetable and invulnerable for the duration but also unable to move, declare basic attacks, cast abilities, use summoner spells, or activate items (20 second cooldown).
- Zhonya's Hourglass: Active: Unique – Time Stop: Put yourself in stasis for 2.5 seconds, rendering you untargetable and invulnerable for the duration but also unable to move, declare basic attacks, cast abilities, use summoner spells, or activate items (120 second cooldown).
Flat Damage Reduction
Several champions have abilities that reduce damage by a flat amount.Some flat damage reductions are factored in after armor or magic resistance. This makes it significantly better the more resistances you have. In addition, periodic damage effects will have each tick of damage reduced. For example, 
Malzahar's 
Malefic Visions ticks 8 times, so 1 flat damage reduction would reduce its total damage by 8.
Flat damage reduction does not work against true damage.
Champions
- Amumu's Tantrum provides 2 / 4 / 6 / 8 / 10 physical flat damage reduction.
- Fizz's Nimble Fighter provides Error: Parameter '3' is not accepted. flat damage reduction against auto-attacks.
Items
- Guardian's Horn: Passive: Unique – Legendary: This item counts as a Legendary item.
