Combat
How Does Preventing Damage Work?
Reduce the next matching damage by the prevention available, keeping any unused amount for the effect's remaining duration.
RiftPal explanation · Answer first
The short answer
PreventReduce the next matching damage a unit would take by the stated amount. Only the amount actually prevented is used; any remainder lasts for the effect's stated duration.View in Glossary reduces the next matching DamageDamage represents harm marked on a unit and stays marked until it is healed.View in Glossary a unit would take by up to the stated amount. Only the amount actually prevented is used; any remainder can apply to later matching damage during the effect's duration.
RiftPal explanation
How it works
PreventReduce the next matching damage a unit would take by the stated amount. Only the amount actually prevented is used; any remainder lasts for the effect's stated duration.View in Glossary ion is a Replacement EffectAn effect that changes an event before it happens, often using "would," "as" or "instead."View in Glossary . Work out the incoming DamageDamage represents harm marked on a unit and stays marked until it is healed.View in Glossary , including Bonus DamageA positive increase to a Deal action. It adds once to a single target, separately to each target dealt an amount, or once to the shared total when damage is split.View in Glossary , then reduce it by the unit's Prevent value. Prevention cannot make damage negative. If it reduces the amount to 0, no damage was dealt, so abilities that require the unit to take damage do not trigger. A value of 'All' remains in place for its stated duration.
Card reference
Ki Barrier
Combat DamageCombat Damage is the Damage units deal as part of Combat.View in Glossary assignment already accounts for prevention. For example, an undamaged 2 MightMight represents both how much Combat Damage a unit can contribute and how much marked Damage it can normally take before being killed.View in Glossary unit with PreventReduce the next matching damage a unit would take by the stated amount. Only the amount actually prevented is used; any remainder lasts for the effect's stated duration.View in Glossary 3 needs 5 combat damage assigned for the result to be Lethal DamageMarked damage sufficient to kill a unit in cleanup. Normally it must be a nonzero amount at least equal to the unit's Might. Effects can change the amount needed.View in Glossary . The prevention effect is accounted for during assignment and is not applied a second time when the assigned damage is dealt.
Spell
Ki Barrier
[Reaction] (Play any time, even before spells and abilities resolve.) Choose a unit. Prevent the next 7 damage that would be dealt to it this turn. (Opponents can assign it extra combat damage to kill it.)
Preventing all damage is different from being unable to be dealt damage. A unit with all damage prevented can still receive a mandatory assignment, but no amount is Lethal DamageMarked damage sufficient to kill a unit in cleanup. Normally it must be a nonzero amount at least equal to the unit's Might. Effects can change the amount needed.View in Glossary . A unit that cannot be dealt damage is ignored for mandatory assignment. If another replacement also changes the same combat damage, the affected unit's ControllerThe player who currently controls a game object and makes the choices assigned to that controller. Control is established differently for battlefields and other objects, and need not match ownership.View in Glossary chooses their order during assignment.
In practice
Example
A unit has protection against the next 3 damage this turn. It would take 2 damage, so both are prevented and 1 prevention remains. Later it would take 3 damage, so the remaining 1 is prevented and the unit marks 2 damage. The protection is now used up.