[RPG] How does the Interception fighting style interact with Uncanny Dodge

class-featurecombatdnd-5ereactions

If an enemy hits a Rogue with an attack, the Rogue uses Uncanny Dodge as a reaction, and at the same time a Fighter uses their reaction to use Interception to lessen the damage the Rogue takes: which feature takes effect first?

Interception
When a creature you can see hits a target, other than you, within 5 feet of you with an attack, you can use your reaction to reduce the damage the target takes by 1d10 + your proficiency bonus (to a minimum of 0 damage). You must be wielding a shield or a simple or martial weapon to use this reaction.

Uncanny Dodge
When an attacker that you can see hits you with an attack, you can use your reaction to halve the attack's damage against you.

So let's say the attack deals 30 damage, and both Uncanny Dodge and Interception takes place. The Fighter rolled an 8 for the Interception. Does the total damage taken becomes:
a. (30 – 8)/2 = 11, or
b. (30/2) – 8 = 7 ?

Best Answer

Because both reactions have the same trigger (the rogue being hit by an attack), the most logical ruling is that they are effectively simultaneous. Xanathar's Guide to Everything gives the following rule for resolving simultaneous effects (on page 77, at the start of the Dungeon Master's Tools chapter):

Simultaneous Effects

Most effects in the game happen in succession, following an order set by the rules or the DM. In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table—whether player or DM—who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first.

This rule suggests that the DM should get to choose the order of the reactions, since the current turn belongs to the monster who was attacking the rogue. They'll probably choose to make the fighter's Interception reaction happen before the rogue's Uncanny Dodge, since that results in more damage, which is in the monster's interest.

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