I was contemplating a rules tweak for Dungeon Grappling, and a backer made a comment that seemed worthy of further consideration, and so I had cause to wonder what reactions could be used for in Fifth Edition.

Ian Borchardt had some time on his hands, so he says, and compiled this list. I asked for permission to repost, and he gave it. Since then, Axel Castilla compiled the list into a downloadable document.

GENERAL

If you perform the Ready action you can prepare any action as a reaction. However you must specify a specific trigger for your reaction and can only take one reaction in a round. When the trigger event happens you can act immediately (or you can ignore the trigger). If casting a spell as a reaction, then the spell must have a casting time of 1 action and you must maintain Concentration until you react.

Remember only 1 reaction a round.

EVERYONE

• Everyone has a trigger of “if foe physically moves out of reach” for an opportunity attack.
• A mounted combatant has the trigger “if mount falls prone” to dismount and land on their feat automatically. [Which is very unrealistic if you ask me… -IB]

CLASS SPECIFIC

The following abilities depend on your class. The entries are presented as options, but some of them (such as with the thief) are class features that all thieves get at the appropriate level.

Barbarian

• A 14th level Barbarian has the trigger “take damage from a creature within 5 feet” to launch a reaction attack. [Retaliation]

Bard

• At 3rd level a Bard of the College of Lore has the trigger “creature that you can see within 60 feet of you makes an attack roll, an ability check, or a damage roll” to use Bardic Inspiration. [Cutting Words]

Cleric

• [Turned undead lose the ability to react.]
• A Cleric of the Light Domain has the trigger “you are attacked
  by a creature within 30 feet of you that you can see” to impose a Disadvantage to the attack. [Warding Flare]
• A 6th level Cleric of the Nature Domain has the trigger “you or a creature within 30 feet of you takes acid, cold, tire, lightning, or thunder damage” to grant resistance to that attack. [Dampen Elements]
• A Cleric of the Tempest Domain has the trigger “hit by an attack by a creature within 5 feet” to do 2d8 lightning or thunder damage (halved with a Dex saving throw). [Wrath of the Storm]
• A 6th level Cleric of the War Domain has the trigger “a creature within 30 feet of you makes an attack roll” to give that creature a +10 to their attack roll by using Channel Divinity. You may see the roll first, but before the DM tells you of it’s success. [War God’s Blessing]

Fighter

• A Fighter with the Protection Fighting Style (and a shield) has the trigger “attacks a target other than you within 5 feet” to give disadvantage to that attack. [Protection]
• A Battle Master Fighter with Commander’s Strike can use an attack and a superiority die to “give a command to a friendly creature who can see and hear them” which acts as a trigger for their ability to make an attack with a bonus equal to the superiority die as a reaction. [Commander’s Strike]
• A Battle Master Fighter with Maneuvering Attack can expend a superiority die to create the trigger “with a successful attack by yourself” to allow a half speed reaction move by a friendly creature that does not attract an opportunity attack from the creature just struck. [Maneuvering Attack]
• A Battle Master Fighter with Parry has the trigger of “damaged by a melee attack” to reduce the damage taken by their Dex bonus and the roll of the superiority die expended in this reaction. [Parry]
• A Battle Master Fighter with Riposte has the trigger of “creature misses you with a melee attack” to make a melee attack with a dmage bonus equal to the superiority die expended in this reaction. [Riposte]

Monk

• A 3rd level Monk with the trigger “hit by a ranged weapon attack” to reduce the damage from the attack by 1d10 + Dex + Level (and if the damage is reduced to 0, they have a free hand, and spend a point of Ki, they may return the attack). [Deflect Missiles]
• At 4th level a Monk with the trigger “falling” to reduce the damage taken by 5 times their level. [Slow Fall]

Paladin

• A Paladin with the Protection Fighting Style (and a shield) has the trigger “attacks a target other than you within 5 feet” to give disadvantage to that attack. [Protection]

Ranger

• A 3rd level Hunter Ranger with Giant Killer has the trigger “after a Large (or bigger) creature within 5 feet attacks you (successfully or not)” to attack. [Giant Killer]
• A 15th level Hunter Ranger with Stand Against the Tide has the trigger “opponent misses you with a melee attack” to force that creature to repeat that attack against another creature (other than itself). [Stand Against the Tide]
• A 15th level Hunter Ranger with Uncanny Dodge can use the trigger “attacker you can see hits you with an attack” to reduce the damage from the attack by half. [Uncanny Dodge]

Rogue

• A 5th level Rogue with Uncanny Dodge can use the trigger “attacker you can see hits you with an attack” to reduce the damage from the attack by half. [Uncanny Dodge]
• A 17th level Arcane Trickster Rogue can use the trigger ” creature casts a spell that targets you or has you in the area of effect” to attempt to negate the spell and steal it. (Limited Use) [Spell Thief]

Sorcerer

• A 6th level Wild Magic Sorcerer can use the trigger “when another creature you can see makes an attack roll, an ability check, or a saving
  throw” to expend 2 sorcery points and add or subtract 1d4 points from the roll. [Bend Luck]

Warlock

• A 6th level Warlock dedicated to The Archfey can use the trigger “when you take damage” to turn invisible and teleport up to 60 feet. (Limited Use) [Misty Escape]
• A 10th level Warlock dedicated to The Archfey can use the trigger “a creature attempts to charm you” to attempt to charm the creature instead [Beguiling Defenses]
• A 6th level Warlock dedicated to The Great Old One can use the trigger “makes an attack against you” to cause the attack to be Disadvantaged. (Limited Use) [Entropic Ward]

Wizard

• A 6th Level Wizard of the School of Abjuration can use the trigger “a creature within 30 feet of you takes damage” to cause their Arcane Ward to absorb the damage instead.
• A 6th level Wizard of the School of Enchantment can use the trigger “a creature you can see makes an attack roll against you” to attempt to divert that attack to another target within range of the attack. (Limited Use) [Instinctive Charm]
• A 10th level Wizard of the School of Illusion can use the trigger “a creature makes an attack roll against you” to cause the attack to be wasted against an illusionary duplicate. (Limited Use) [Illusionary Self]

FEATS

• A character with the Defensive Duellist feat can use the trigger “hits you with an attack” to add their Proficiency Bonus to their AC, provided they are armed with a finesse weapon.
• A character with the Mage Slayer feat can use the trigger “a creature within 5 feet of you casts a spell” to make a melee attack against that creature.
• A character with the Sentinel feat can use the trigger “a creature within 5 feet attacks someone else” to make a melee attack against that creature.

SPELLS

These spells may be cast as a reaction

• Counterspell has the trigger “when you see a creature within 60′ casting a spell.”
• Feather Fall has the trigger “when you or a creature with 60′ falls.”
• [A familiar may use it’s reaction to deliver a spell you cast.]
• Hellish Rebuke has a trigger “when you take damage from a creature within 60′ that you can see.”
• Shield has the trigger “when you are hit by an attack or targetted by a magic missile spell.”

FROM THE MONSTER MANUAL

Beholder (Spectator)[Spell Reflection]

Trigger: Spectator saves vs spell or a spell attackmisses it.
Reaction: Spell now targets another creature spectator can see within 30 feet.

Death Knight[Parry]

Trigger:Melee attack would hit (and Knight has melee weapon and sees attacker)
Reaction:Knight increases AC by +6.5

Demon (Marilith)[Parry]

Trigger:Melee attack would hit (and Maralith has melee weapon and sees attacker)
Reaction:Maralith increases AC by +5.The Marilith my take one Reaction on every turn in combat. [Reactive]

Devil (Chain Devil)[Unnerving Mask]

Trigger:Devil sees creature within 30 feet.
Reaction:Devil looks like a departed loved one or bitter enemy (illusion). Creature must make DC 14 Wisdom save or be frightened for 1 turn.

Devil (Erinyes)[Parry]

Trigger:Melee attack would hit (and Erinyes has melee weapon and sees attacker)
Reaction:Erinyes increases AC by +4.

Drow (Elite Warrior)[Parry]

Trigger:Melee attack would hit (and Drow has melee weapon and sees attacker)
Reaction:Drow increases AC by +3.

Giant (Stone)[Rock Catching]

Trigger:Rock (or similar object) hurled at giant
Reaction:Giant makes DC 10 Dexterity save to catch rock and take no damage.

Gnoll (Pack Lord)[Incite Rampage]

Trigger [Action]:One creature with Rampage trait within 30′ that can hear Gnoll. [Recharge]
Reaction:Creature may use its Reaction to make an immediate melee attack.

Goblin (Boss)[Redirect Attack]

Trigger:Creature Goblin sees targets it with an attack.
Reaction:Swap spaces with a goblin within 5′ (who becomes the target of the attack).

Half Dragon (Red Dragon Veteran)[Parry]

Trigger:Melee attack would hit (and veteran has melee weapon and sees attacker)
Reaction:Veteran increases AC by +3.

Hobgoblin (Warlord)[Parry]

Trigger:Melee attack would hit (and Warlord has melee weapon and sees attacker)
Reaction:warlord increases AC by +3.Hydra[Reactive Heads]For each head beyond the first, Hydra gets an additional
Reaction for opportunity attacks only.

Kuo-Toa[Sticky Shield]

Trigger:Creature misses Kuo-Toa with melee attack.
Reaction:Attacker makes DC 11 Strength save or weapon become stuck to shield.

Ooze (Black Pudding)[Split]

Trigger:Medium or larger Black Pudding with 10+ hp takes Lightning or Slashing damage.
Reaction:Split into two new puddings (each has half hp and one size smaller).

Ooze (Ochre Jelly)[Split]

Trigger:Medium or larger Ochre Jelly with 10+ hp takes Lightning or Slashing damage.
Reaction:Split into two new jellies (each has half hp and one size smaller).

Shield Guardian[Shield]

Trigger:Creature attacks amulet wearer and guardian is within 5 feet.
Reaction:Increase amulet wearer’s AC by +2

NPC (Bandit Captain)[Parry]

Trigger:Melee attack would hit (and bandit has melee weapon and sees attacker)
Reaction:Bandit increases AC by +2.

NPC (Gladiator)[Parry]

Trigger:Meleeattack would hit (and gladiator has melee weapon and sees attacker)
Reaction:Gladiator increases AC by +3.

NPC (Knight)[Parry]

Trigger:Melee attack would hit (and knight has melee weapon and sees attacker)
Reaction:Knight increases AC by +2.

FROM VOLO’S GUIDE TO MONSTERS

Goliath[Stone’s Endurance]

Trigger:Goliath takes damage. [Limited]
Reaction:Reduce damage taken by d12 + Con bonus.

Gnoll (Witherling)[Vengeful Strike]

Trigger:A gnoll within 30′ is reduced to 0 hp.
Reaction:Witherling makes melee attack.

Morkoth[Spell Reflection]

Trigger:Morkoth saves vs spell or a spell attack misses it.
Reaction:Spell now targets another creature Morkoth can see within 120 feet.7

Nilbog[Reversal of Fortune]

Trigger:Another creature does damageto Nilbog.
Reaction:Damage reduced to 0 and Nilbog regains 1d6 hp.

Orc (Blade of Ilneval)[Ilneval’s Command]

Trigger:Blade takes action to issue command. [Recharge]
Reaction:Up to three allied orcs within 120′ can use
Reaction to make a melee attack.

Orc (Tanarukk)[Unbridled Fury]

Trigger:Being hit by melee attack.
Reaction:Tanarukk makes melee attack with Advantage against attacker.

NPC (Enchanter)[Instinctive Charm]

Trigger:Creature Enchanter sees and within 30′ initiates an attack against Enchanter.[Limited]
Reaction:Attacker makes DC 14 Wisdom save or attacks creature closest to itself (other than Enchanter or itself).

NPC (Martial Arts Adept)[Deflect Missile]

Trigger:Martial Artist hit by ranged attack.
Reaction:Reduce damage by d10 +3.If damage reduced to 0 missile caught if free hand.

NPC (Master Thief)[Uncanny Dodge]

Trigger:An attack hits thief. Thief must see attacker.
Reaction:Thief take half damage from attack.

NPC (War Priest)[Guided Strike]

Trigger:Creature within 30′ (including self about to make an attack roll). [Limited]
Reaction:+10 bonus to attack roll.

NPC (Warlock of the Archfey)[Misty Escape]

Trigger:Warlock takes damage. [Limited]
Reaction:Warlock turns invisible (until start of next turn or attack or cast spell) and teleports to an unoccupied space within its sight.

NPC (Warlord)[Command Ally Legendary Action]

Trigger [Action by Warlord]:One ally within 30′ that can see and hear Warlord.
Reaction [by ally]:Ally makes weapon attack with Advantage as a
Reaction.