DnD 5e: How Reactions Work

Combat situations in Dungeons & Dragons can get hairy quickly, and though they sometimes seem to drag on forever, 10 rounds of combat is actually just one minute. Composed of actions, bonus actions and reactions (and sometimes even lair actions), there's often a lot going on during combat, and sometimes it can get a little confusing.

Particularly against major bosses, players need to maximize their potential in every round of combat, and that means taking reactions when they make themselves available. A reaction is exactly what it sounds like: When a situation presents itself, the player may be able to react to that situation, sometimes even taking extra attacks, minimizing damage on an incoming attack or even negating an attack or effect before it can take hold.

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When Can Players Use Reactions?

All players, including the Dungeon Master controlling enemy monsters and creatures, get one reaction per character each round. Once that reaction has been used, another one can't be taken until combat reaches the top of the initiative again. One of the most common reactions in combat is the attack of opportunity, which occurs when an enemy combatant attempts to move away from melee range or passes within melee range on their way to safety.

An enemy could potentially pass by multiple combatants while attempting to flee, and so long as those combatants haven't used their reaction for the round, each of them could take an attack of opportunity against the enemy. The only time this doesn't work is when the creature uses the disengage action (or a similar class feature) to make a clean getaway. Typically, players can only make a basic melee weapon attack when making an opportunity attack. The exception to that rule is when a player who has taken the War Caster feat is given the option to make a single target spell attack with one action casting time instead.

Reactions can also be used to prepare for a specific situation. For example, if enemy has hidden or gone invisible (making attacking nearly impossible), a player can use their action to prepare a reaction, such as a melee or spell attack, that commences instantly when the stated criteria -- the enemy reappearing -- is achieved.

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How to Acquire Special Reactions

All players have access to a small number of basic reactions, like attack of opportunity and readied action, as well as the dismount reaction and spell identification. There are plenty of others that are incredibly useful once they are learned. Some can be acquired through class or subclass progression, from taking on a feat or using a reactionary spell.

Rogues, for example, gain access to the Uncanny Dodge reaction at level five, which allows them to half the damage on an attack they can see coming. Barbarians who follow the Path of the Berserker can use their reaction to make a melee attack against a creature within five feet that's caused them damage. Some Fighters can use their reaction when holding a shield to protect a nearby ally, forcing disadvantage on attack rolls against that ally.

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Some spellcasters can cast Counterspell as a reaction. When someone takes an unexpected fall, casters can use the Feather Fall spell as a reaction to protect them from taking major damage, or one could call up a magical shield to protect themselves from an incoming attack. A caster may even throw up a Mental Barrier to protect their thoughts from being invaded when being forced to make a Wisdom, Intelligence or Charisma saving throw.

In addition to War Caster, there are other feats that provide reactions. Gnomes who take the race-specific Fade-Away can turn invisible after taking damage, allowing them to get out of the line of fire and assess their injuries without the pressure of another attack. Halflings gain access to Second Chance, which allows them to use their reaction to force an enemy to make attacks at disadvantage against them. Taking advantage of every opportunity to inflict or negate damage in combat not only speeds up combat, but it can also be essential to keeping the party alive and adventuring.

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