[RPG] Is the Undead Fortitude trait’s DC based on damage dealt or damage taken

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I'm new to DMing and I'm looking through options to throw at my players. I see the Zombie creature has the following trait:

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Do I set the DC to 5 + the damage that was rolled against it, or do I set it to 5 + the health it had before dropping to 0? I've not found any rulings on the matter.

Best Answer

If the zombie has resistance or vulnerability to the attack, the DC is calculated by the final damage amount.

From PhB: Resistance and Vulnerability

For example, a creature has resistance to bludgeoning damage and is hit by an attack that deals 25 bludgeoning damage. The creature is also within a magical aura that reduces all damage by 5. The 25 damage is first reduced by 5 and then halved, so the creature takes 10 damage.

The amount of damage the zombie takes (which is the input value to Undead Fortitude) is the result of the final damage calculation, after all modifiers.

However, the DC is independent of the zombie's remaining HP

From the question:

Do I set the DC to 5 + the damage that was rolled against it, or do I set it to 5 + the health it had before dropping to 0? I've not found any rulings on the matter.

Creatures can take damage beyond their current hit points - but the result of that damage is normally to set their HP to zero.

From PhB: Massive Damage

For example, a cleric with a maximum of 12 hit points currently has 6 hit points. If she takes 18 damage from an attack...

If damage taken was capped at current hit points, it would not be possible for the cleric to take 18 damage from an attack in that example. Similarly, the damage a zombie takes is not capped at its current hit points either. If it takes 15 damage from an attack, the save DC is 20 regardless of whether it had 15 HP or 1 HP before.

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