Damage modifier

Damage modifier refers to the buffs or debuffs imposed or granted to a unit that causes it to deal or receive increased or decreased damage. These are percentual values, they don't add any extra flat damage but modify all damage normally done. These buffs or debuffs are only recevied by champion abilities, on-hit effects from items and some masteries.

Damage dealt modifier
These are buffs that allow the champion that have them to deal increased or decreased magic or physical damage from autoattacks, abilities, item's actives and. 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. So if 's uses an autoattack on a stunned champion just after using a maxed on it, while affected by, his autoattack's damage will be modified by 107.525% (110%*115%*85%).

Champion abilities
Note: Only the armor buff effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
 * blasts her target and slows it for 4 seconds. While the target is slowed, Kayle inflicts 6 / 7 / 8 / 9 / 10 % more damage to the target from all sources.
 * allows her to focus intensely on one single target for 6 / 7 / 8 seconds to deal 20 / 30 / 40 % increased damage to it from all sources.
 * allows him to deal 20% increased damage when striking a unit from behind with his autoattacks or abilities.
 * afflicts his target, dealing damage to them over time and causing Swain to deal increased damage to it for 4 seconds from all sources.
 * allows his autoattacks to deal 10% increased damage if his target is slowed, stunned, snared or suppresed.
 * allows him to blink to his target and deal 3 / 6 / 9 / 12 / 15 % increased damage.

Masteries

 * increases your champion's physical and magical damage by 4%.

Decreasing damage dealt

 * enhances her, to debuff her target to deal 20% less damage for 4 seconds.
 * makes his autoattacks reduce all damage that his target deals by 15% for 2.5 seconds.

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, item's actives and. 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.

Decreasing damage received

 * grants him 50 / 60 / 70 % damage reduction for 7 seconds.
 * grants him 50% damage reduction for 2 seconds
 * places a defensive shield on himself, decreasing all damage taken by a 20 / 24 / 28 / 32 / 36 % for 3 seconds.
 * channels for 1 second and then receives 10 / 12 / 14 / 16 / 18 % reduced damage for 20 seconds.
 * reduces all magic damage taken by 10 %.
 * enhances to grant her 15 / 20 / 25 / 30 / 35 % damage reduction for 3 seconds after blinking.
 * creates a great vortex of natural energy on a nearby area reducing non-tower damage done to allied champions in the area by 20% until toggled off.
 * grants him 5% damage reduction for each ghoul summoned.