In the MM a zombie has the following property:
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.
Is "damage taken"…
- the total damage since the start of the fight (which makes it impossible to pass, as the first DC is equal to 5+22+damage taken),
- total damage since first use of this property (which makes most sense to me, as a zombie becomes weaker the more damaged it is), or
- the current attack's damage (which makes a zombie a very difficult opponent for a 1st level party)?
Yesterday I ruled the third way, and four zombies wiped the floor with the party, as most attacks of my party did a mere 4–5 points of damage with no crits or radiant damage from anybody, which made it DC 9 and easy to pass. It took five turns before one almost-dead zombie failed the check.
Best Answer
It’s the damage that reduced the zombie to 0 hit points.
You were applying the ability correctly. The phrase damage taken is referencing the beginning of the sentence If damage reduces ....
Note that four zombies (CR 1/4) is a deadly encounter for four 1st-level characters, so the outcome you describe is as expected. Of course, you can always fudge things in combat if you feel like the encounter you designed was much tighter than you intended.