Debuff

A debuff is the inverse of a buff. It is an effect that hampers the target in some way for a duration. Among other effects, debuffs usually lower attributes, hinder the effectiveness of the target's actions, or cause damage over time.

Similarly with buff, certain debuff are stackable: repeated application on the same target will causes a stacking effect of increased effectiveness. Non-stackable debuffwill only be refreshed by repeated application. All debuffs are temporary, and will not persist after the affected target's death.

There are currently 3 sources of debuffs: champion abilities, summoner spells, and item active effects.

Champion Abilities
Majority of the champions possesses a certain type of debuff. Such debuff can be armor reduction from, magic resist reduction from , or attack damage reduction from.

A debuff that reveals its victim to the enemies may cause other wards and stealthed units near the victim to be reveal as well. Currently and  works 100% of the time. and have unconfirmed effects for the first, while the latter being less consistent.

Damage over time, and Direct CC such as will not be included. Crowd control, which is also a form of debuff by default and covered by crowd control section.

Summoner Spells
Summoner spells have greater effect as a debuff as compared to champion abilities.
 * reduces the damage dealt from the affected champion and slow it greatly.
 * deals true damage over 5 seconds, and reducing all healing and regeneration effects on the affected target for the duration.

Item Effects
Certain items possesses capability to debuff enemy. Some of it will not debuff enemy unless the active component was utilized, some will automatically do so when certain action was fulfilled (such as autoattack, casting spells etc), and some may possesses both components.

Active Items
These items does not have a passive effect of debuffing enemy on its own, but possesses an active component that will debuff enemy.
 * active effect will deal magic damage and slow its target's movement speed by 50% for 3 seconds.
 * active effect will deal magic damage and slow its target's movement speed by 50% for 3 seconds.

Passive Item
Items such as the following will debuffs enemies automatically when they comes into range. No activation is required to bring the effect into place.
 * passively reduces all nearby enemy champions' magic resist by 20.
 * passively slows all nearby enemies' attack speed by 20%.
 * passively allows auto-attacks to slow your target's Movement Speed by 40% for 2.5 seconds. (30% for ranged attacks)
 * inflict 20 magic damage on-hit and reduces the target's magic resistance by 6 for 8 seconds. Stacks up to 4 times.
 * gives auto-attacks 25% chance to slow the target's Movement Speed by 30% for 2.5 seconds.
 * passively causes your spell damage to additionally slows the target by 15% or 35% for 1.5 seconds.
 * allow auto-attacks to reduce your target's armor by 15 for 5 seconds. Stacks up to 3 times.
 * allows auto-attacks to have a 25% chance to slow your target's movement speed by 35% for 2.5 seconds.
 * causes user to have 20% chance on being hit by an auto-attack to reduce attacker's movement speed and attack speed by 35% for 3 seconds.

Items with both components
These are items with both a passive and active component that both debuffs the enemy.
 * inflict 4 damage per second on-hit for 8 seconds. Upon activation, auto-attacks temporarily apply Grievous Wound which reduces healing and regeneration effects by 50% for 8 seconds.
 * grants the wielder 20% chance on being hit by an auto attack to slow the attacker's movement and attack speed by 35% for 3 seconds. Upon activation of its active component, it will slows movement speed and attack speed of surrounding enemy units by 35% for 2 seconds + 0.5 seconds for every 100 armor and magic resistance.