The Tachyons were a species of six-limbed, telepathic extraterrestrial that landed on Earth sixty-five million years ago, around the extinction of the dinosaurs. They stayed alive long enough to breed the Reptilians and Cryptocleidus before going dormant. In the present day, they were awakened when a team of scientists, Dr Alexander Preloran and his assistants Dr Hans Sopler and Dr Ted Hoffman, find their ship, the Leviathan, stuck at the bottom of the ocean. While investigating, however, they discover that the alien crew is still alive.
Soon afterwards, H.E.A.T. encounters a transmission coming from the aliens, and track the source in the Hazaard Abyss. They furthermore discover that the team of scientists never returned from their expedition, and so investigate the situation. When they dock with the Leviathan, they find the crew alive, and that they have taken Dr. Preloran as their mental captive. Having used their telepathic abilities to absorb the scientists' knowledge, they decide to colonize Earth. After Nick blows up the Leviathan's controls, Preloran breaks free of the Tachyons' control, and sacrifices himself to let the others escape. The ship then explodes, killing all on it.
However, it is revealed later on in the Monster Wars trilogy that the aliens actually downloaded their minds onto Preloran's two assistants, Drs. Sopler and Hoffman, who gradually mutate into the forms of the Tachyons. The two use their new psychic abilities to keep H.E.A.T. divided and the military on their side, and then use the technology to resurrect Zilla Sr. as a cyborg, Cyber-Zilla. The cyborg attacks Nick and H.E.A.T., but when Zilla Jr. comes to the rescue, he is torn between his old parent and Tatapolis. The Tachyons use this confusion to take control of him, and they send the mutations across the world to attack major cities. This way, the military forces will be weakened enough to be unable to resist the approaching Tachyon fleet. However, Dr. Mendel Craven manages to scramble the signal that is controlling the monsters, and they destroy enough of the Tachyon craft to force a retreat. Zilla Jr. then, after finally siding with Nick, fights his cyborg parent, tearing off his right cybernetic arm and tearing off his chest plate. Hans Soplar and Ted Hoffman then begin to de-mutate back into their human forms.