Zombie hulks are what resulted after a creature with one of the strange devices behind their ear died from natural causes. The transformation from dead guy to zombie hulk was never observed directly, but does not occur immediately after death. The man who was tied to the fifth stone slab at the beginning of the adventure died in front of the party but was not revealed to be a zombie hulk until three days later when the gang returned to Shadowspar Keep.
The strange devices may also explode inside a creature's brain before death. It is assumed that the creature would not then turn into a zombie hulk due to the fact that both his brain and the device are destroyed in the process.