The death ward spell states:
[…] The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends.
If the spell is still in effect when the target is subjected to an effect that would kill it instantaneously without dealing damage, that effect is instead negated against the target, and the spell ends.
The rules section on "Instant Death" states:
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum […]
Thanks to user @Xirema for showing that we can combine this with the section on Death Saving Throws, which states:
Damage at 0 Hit Points. If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death.
Thus we know that instant death applies even while at 0 HP.
That said, I'm unsure what happens in the following scenario:
A creature has 0 hit points and death ward is cast on them. They then take damage equal to double their maximum HP.
Does the creature die outright? Do they live but with 0 HP and the spell ends? Do they somehow now have 1 HP and the spell ends? How do these interact with each other?
Best Answer
Death Ward has no effect
For the first part of the spell description, this covers the situation for when a creature drops down to 0 health. Since the creature is already at 0 health, they aren't dropping to it, so the spell has no effect.
For the second part of the spell description, this isn't applicable to the situation, since damage is being dealt. The purpose of that section is to protect against spells like Power Word Kill, which instantly kills without dealing any damage.
Since damage is being dealt and the creature is already at 0 in the situation you presented, Death Ward would do nothing for them.