Their presence is signaled by a foggy, radioactive mist cloud and an intense cold. Though nothing is known about the creatures' origin and motives, Professor Crevett theorizes that they might have come from a dying planet, seeking to colonize other worlds; he also acknowledges their preference for mountains as indicative of different atmospheric requirements, and believes the cold foggy cloud represents the aliens' attempt to reproduce their own environment; possibly even the beginning of an attempt to "xenoform" Earth.
The aliens from Trollenberg also show the ability to control people, putting them in a "living dead"-like state. Apparently, the aliens feel threatened by humans that possess telepathic abilities, and will hypnotize others to kill them. Fortunately, the Trollenberg invaders are also vulnerable to fire. With the protagonists and villagers safe inside the heavily fortified observatory; military planes are ordered to bomb the area, successfully killing the alien menace.