How buffs and debuffs work · Raid Optimizer

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In short

Every buff and debuff lasts a number of its bearer's own turns. Most lose a turn at the end of that turn. [Poison] and [HP Burn] tick and lose a turn at the start.
An effect placed during its bearer's own turn does not lose a turn on that turn, so an N-turn effect covers N of the bearer's turns.
Of one kind, only one counts: a stronger one replaces a weaker one, a weaker one is refused, and for equal strength the longer one wins. Poison stacks (up to 10); a Shield does not.
A champion or boss holds at most 10 debuffs. A new one is refused when the bar is full, but one already on it can still be refreshed.
[Block Damage] stops all damage, [Unkillable] only stops death, [Shield] soaks damage until it is used up.

One clock: the bearer's own turns

A buff helps the champion carrying it and is usually placed on allies; a debuff hurts the bearer and is placed on enemies. Both come with a duration in turns, and the turns that count are the bearer's own, not rounds of the battle. A 2-turn buff on a slow champion lasts longer in real time than the same buff on a fast one.

When effects lose a turn

When	Which effects
At the end of the bearer's turn	Nearly everything: [Stun] and the other control debuffs, [Increase ATK], [Decrease DEF], [Weaken], [Block Damage], [Unkillable], [Shield], [Counterattack], [Revive on Death], [Block Debuffs]
At the start of the bearer's turn	[Poison] and [HP Burn]: the damage lands and the effect loses a turn. [Continuous Heal] also heals at the start of the turn.

The game decides this per effect, so a few unusual ones differ; when it matters, trust the skill text. What we measured: poison lost a turn at the start of the bearer's turn in 219 of 219 observed turns and never at the end, while [Decrease DEF], [Weaken] and [Decrease ATK] lost theirs at the end every time. A [Poison] placed for 3 turns ticks exactly 3 times (we counted 988 finished poisons), and since the last tick comes at the start of a turn, an expiring poison is already gone when its bearer acts.

Effects placed during the bearer's own turn

An effect that lands while its bearer is taking a turn does not lose a turn when that turn ends. It starts counting from the bearer's next turn. We saw this on debuffs placed on the boss during its own attacks: all 6 survived the end of that turn. The result is that an N-turn effect always covers N of the bearer's turns, however it got there.

A 2-turn buff on you...	Your next turn	The turn after
...cast by an ally during their turn	Active; 1 turn left at the end	Active; gone at the end
...cast by you on yourself during your turn	Active; 1 turn left at the end (no turn lost on the casting turn)	Active; gone at the end

A 1-turn buff you place on yourself, such as [Block Damage], therefore protects you against everything the enemy does until the end of your next turn.

Careful A buff placed on an ally is only as good as the order of turns. If the ally's turn comes before the enemy's big attack, a 1-turn buff on it ends at the end of that turn and is gone when the attack arrives. This is why speed matters; see Turn meter and speed. A stunned or frozen turn still counts as a turn for the clock; see Skills, cooldowns and auto-battle.

Stacking and replacing

Effects come in kinds, and each kind usually has two strengths, such as [Increase DEF] 30% and 60% or [Decrease SPD] 15% and 30%. Of a kind, only one counts at a time. When a new one arrives while another of that kind is on the bearer, the game compares them:

The new effect is...	Result
Stronger (for example 60% over 30%)	Replaces the old one
Weaker	Refused; the old one stays
Equal in strength, and lasts longer (or has no end)	Replaces the old one, so it acts as an extension
Equal in strength, same or shorter duration	Refused (an exact tie only refreshes during the bearer's own turn)

Different kinds do not interact: [Decrease DEF] and [Weaken] sit side by side. For the stat effects, an increase and a decrease of the same stat add their percentages: +30% SPD and −15% SPD is ×1.15, and the Clan Boss's ATK works the same way.

Some effects are built to stack. [Poison] can have up to 10 copies on one target and every copy ticks, [Continuous Heal] can be placed more than once, and a unit can carry several [Bomb]s, each with its own timer. [HP Burn] is the opposite: only one per bearer.

The Shield rule

A champion holds one skill [Shield] at a time (the Shield gear set's shield is kept separate). A new Shield is compared with what is left of the old one, not with its starting size:

A bigger new Shield replaces the old one. A smaller one is refused, so a weak Shield cannot knock out a strong one you are relying on.
A Shield in its last turn, or one about to expire at the end of its bearer's current turn, gives way to a new one even if the new one is smaller. We saw a fresh Shield replace an expiring one in 40 of 40 cases.

A recast of the same skill is the same size as the one it replaces, so it simply renews it.

The limit of 10

A unit holds at most 10 debuffs. We watched the Clan Boss's bar: it never went above 10, a debuff already on it could still be refreshed while the bar was full, and new debuffs such as [Decrease ATK] were not placed at 10. Every Poison copy counts, so ten poisons fill the whole bar. A team that stacks poisons first can leave no room for a fresh [Decrease DEF] or [Weaken]. The game's own constants give the same limit of 10 for buffs, though we have not watched it bite. This check comes last, after the chance, block and resist checks described in Landing debuffs.

Removing, stealing, spreading and extending

Effect	What it does	Good to know
Remove Buffs, Steal Buffs	Strip a number of random buffs from an enemy, or steal them.	The target can resist with RES. Protected buffs cannot be taken.
Remove Debuffs	Cleanse debuffs from allies.	No roll. Debuffs whose text says "cannot be removed" stay.
Spread Buffs, Spread Debuffs	Copy effects to every ally or every enemy.	Some kinds cannot be copied: buffs such as Veil and Taunt, debuffs such as Stun and Deathbrand.
Extend and Shorten	Add or remove turns on buffs or debuffs.	Block Damage, Unkillable, Revive on Death and Taunt cannot be extended. Shortening an enemy's buffs can be resisted.

Protected effects

Some effects are protected: the text says they "cannot be removed, stolen, or transferred". Protection also covers being spread or having the duration changed. Most protection only holds against the enemies of whoever placed the effect: a protected buff cannot be stripped from you by the enemy, and a protected debuff cannot be cleansed by its victim's team, but the placer's own side can still move or remove it. A few effects are protected against everyone. The clearest case is the permanent [Block Revive] the Clan Boss puts on every champion it kills from its 50th turn: nothing on the field can take it off.

Block Damage, Unkillable and Shield

These three get mixed up. They do very different things to the same hit:

	Block Damage	Unkillable	Shield
What it stops	All damage, from every source, including Poison, HP Burn and hits passed on by Ally Protection	Death only: HP cannot fall below 1	Damage, until the Shield is used up or expires
Damage lands?	No	Yes, in full	Partly: the Shield takes it first

Neither Block Damage nor Unkillable stops debuffs: a champion under either can still be stunned or poisoned. Block Damage is checked first, so a hit it blocks does not use up a Shield. Here is one hit of 80,000 (already after DEF) on a champion with 50,000 HP:

Protection	Result
None	The champion dies.
[Unkillable]	The full 80,000 lands; the champion is left on 1 HP.
[Shield] of 40,000	The Shield soaks 40,000 and is gone; the champion keeps 10,000 HP.
[Block Damage]	No damage at all: still 50,000 HP, and any Shield is untouched.

Note Some attacks are written to ignore these buffs. The Clan Boss ignores both [Block Damage] and [Unkillable] from its 50th turn onward. A target under [Hunter's Gaze] loses its Unkillable, Block Damage, Shield and more against your hits. Always read the attacker's text, not just your own buff.