Drop to 0 effects on unconscious creature

[Avonar]

Does anyone know what would happen if a creature already on 0HP suffers an effect that has consequences for dropping it to 0?

There's an effect in Dead in Thay (No spoilers) that can cause damage, and if this damage reduces something to 0HP, it kills them automatically. What would happen if this triggers in something that is already unconscious on 0 HP?

[Unoriginal]

Does anyone know what would happen if a creature already on 0HP suffers an effect that has consequences for dropping it to 0?

There's an effect in Dead in Thay (No spoilers) that can cause damage, and if this damage reduces something to 0HP, it kills them automatically. What would happen if this triggers in something that is already unconscious on 0 HP?

If you take damage while at 0HPs, a number of your death saves auto-fails, and if you were stabilised you're in peril again.

If an effect with a "if those damages drop you to 0HPs, it does X" clause is applied to someone who already is at 0HPs, then you can treat it as if the effect's damage put them at 0HPs again. Like if you got attacked wit a sword while recovering.