Quote Originally Posted by SePhO_zO
Dragon Zombie's randomly cast Pandora's Box upon the depletion of their HP. That means the first Dragon Zombie you fight may not use it, but the second may, and vice versa. If you kill a Dragon Zombie and it doesn't use the attack, fight another one.
Either I have rotten luck or the odds of Dragon Zombies casting Pandora's Box goes way down after the first time. I had one game where I didn't get Pandora's Box from the first one and went back and fought dozens of them and they never used it in the latter battles.

I eventually gave up and started a new game; the first Dragon Zombie I met promptly cast it before dying. So it may well be randomly cast, but the Dragon Zombies seem to behave differently after the first encounter At least in my experience.