[RPG] How does the Great Weapon Master feat’s bonus-action attack work against a zombie that survives thanks to Undead Fortitude

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I was thinking about the Great Weapon Master feat (PHB, p. 165), part of which reads:

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

And the Undead Fortitude trait of Zombie creatures (Monster Manual, p. 316):

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.

If a PC with the GWM feat drops the Zombie to 0 hit points with an non-crit, non-radiant damage attack and the Zombie succeeds on the saving throw, does the PC get another attack as a bonus action?

Best Answer

You don't get a second hit

Undead Fortitude says (emphasis mine):

On a success, the zombie drops to 1 hit point instead.

This indicates that Undead Fortitude interrupts damage. Similar to damage reduction, Undead Fortitude activates after the hit but before damage is taken. Thus, the undead never reaches 0 HP if it passes the Constitution Save, and the GWM bonus attack isn't activated.

Order of events:

  1. You make a lethal hit
  2. The undead makes a save
  3. It's HP becomes 1
  4. GWM isn't activated.

If the ability said something similar to "the undead recovers 1 HP on a success" then he would have dropped to 0 HP and triggered GWM.

Compare to Shield and the Monk's Deflect Missiles:

Shield:

1 reaction, which you take when you are hit by an attack
...
Until the start of your next turn, you have a +5 bonus to AC, including against the triggering [a]ttack.

Deflect Missiles:

you can use your reaction to deflect or catch the missile when you are hit by a ranged weapon [a]ttack.

Both of these abilities interrupt their trigger and have the potential to prevent the incident that triggered them.

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