D&D 5E Instant death and undead fortitude question

[Tush Hog]

Instant Death and undead fortitude work off of a damage total from an attack.

My question is what do you constitute as an attack if a creature or character has multiple attacks in a round. Do you only use the last attack that dropped them or do you add up all the attacks damage if there was a focused attack?

 

[jadrax]

Undead Fortitude. If damage reduces the zombie to 0 hit points, it can 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.

Seems to me that every time the Zombie takes damage this ability would come into play. So a Fighter attacking it four time when it was on 1 Hit Pint would require the Zombie t make 4 saves. If this was not the case, the bit about Critical Hits would not make much sense as that happens on an attack by attack basis.

 

[Tormyr]

Each separate attack is a separate save. Even magic missile, which has 3 or more die rolls, is one attack, and the magic missile text also says they hit simultaneously. So only one save there.