Curse of Strahd question (Spoilers Inside)

[Galithar]

Strahd has the ability to auto transform to mist if he hits 0HP. This ability fails if he is in running water or sunlight. What if the running water or sunlight is applied to his mist form AFTER the transformation, while he is at 0 HP? Would this instantly destroy him?

[Sigreid]

Not sure if it's instant but yes, they can destroy him if he's in most form. Our group got him by trapping him in a force wall box with a character armed with the sun sword.