What is the interaction between "Touch of Death" and "Undead Fortitude"

Question

The "Way of the Long Death" monk has a feature called "Touch of Death":

Touch of Death

Starting when you choose this tradition at 3rd level, your study of death allows you to extract vitality from another creature as it nears its demise. When you reduce a creature within 5 feet of you to 0 hit points, you gain temporary hit points equal to your Wisdom modifier + your monk level (minimum of 1 temporary hit point).

(Bold added for emphasis). How does this work when a WotLD Monk reduces a zombie to 0 hit points and the blow is shrugged off by undead fortitude?

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.

I could see this working one of two ways:

1. A zombie takes a hit, and drops to 0 hit points. Then Undead Fortitude kicks in and on a success, the zombie comes back to 1 HP. The zombie had been reduced to 0 hp, so ToD activates.

2. A zombie takes a hit, and makes a roll to see whether it drops to 0 or 1 HP. On a success, it drops to 1 HP and had never dropped to 0, therefore ToD does not activate.

Is there any official ruling on this case? If not, what is a reasonable interpretation of these rules?

Answer 1

I would rule that Touch of Death does not trigger if Undead Fortitude is triggered.

Touch of Death activates when:

you reduce a creature within 5 feet of you to 0 hit points.

But Undead Fortitude says:

the zombie drops to 1 hit point instead.

Instead here indicates that the zombie drops to 1 hit point instead of dropping to 0, so the trigger for Touch of Death never happens. Or to put it another way, the Zombie drops to 1 hitpoint instead of [the trigger for Touch of Death], since "drops to 0 hit points" is the trigger for Touch of Death.

I would rule this way because Undead Fortitude appears to make it so that the trigger for Touch of Death never happened. There is room to rule otherwise though, as it still requires the DM to make an aparently arbitrary choice about the resolution order. Alternatively, we have some optional guidance...

An optional rule from Xanathar's Guide can change my ruling.

In Xanathar's Guide, we see an optional rule for resolving simultaneous effects:

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.

In this case, if it is the Monk's turn, they may choose to resolve Touch of Death before undead fortitude.

Answer 2

Undead Fortitude and Touch of Death both trigger

Both Undead Fortitude and Touch of Death specify the circumstances under which they activate.

When you reduce a creature within 5 feet of you to 0 hit points

and

If damage reduces the zombie to 0 hit points

It is paradoxical to rule that Undead Fortitude prevents the Zombie from ever having been reduced to 0 hp because, if that were the case, then the feature would prevent its own trigger from occurring.

This can be compared to the Relentless feature of the Wereboar (MM, 209):

if the wereboar takes 14 damage or less that would reduce it to 0 hit points, it is reduced to 1 hit point instead.

Since this triggers on the damage rather than on being reduced to 0 hp it would prevent Touch of Death from triggering.