The Tommyknockers are the titular main antagonists in Stephen King's novel of the same name. A massive spaceship is discovered in the earth and uncovered by Roberta "Bobbi" Anderson, there emerged the eponymous Tommyknockers who are a monstrous alien race that possess technological prowess, but are hindered by their species-wide bout of psychosis. They have a corrupting influence on the nearby town of Haven, although their effects and appearances vary in both the novel and its 1993 miniseries adaptation.
Within the original book, the Tommyknockers have since perished as a result of the crash, caused by squabbles and fights among the crew. However, as the spaceship is unearthed, their influence corrupts the nearby town of Haven. This is caused by a radioactive gas emitted from the spacecraft that begins with basic symptoms such as bleeding and losing teeth. However, many of the townspeople are gradually transformed into the likeness of the Tommyknockers themselves, some gaining transparent skin and tentacles in place of their genitals. This poisonous influence grants the residents with their technological prowess, often making fantastical adjustments to household items and appliances that are powered by batteries (such as Anderson's water heater powered by a small nuclear reactor). However, the Tommyknockers' psychosis and violent tempers also carry onto the populace, prompting some residents to take violent measures against outsiders to ensure security and even against each other (such as one resident who uses a teleportation/disintegration machine to eliminate a member of his poker circle for cheating him).
Anderson's friend and ex-lover James "Gard" Gardener arrives in Haven after picking up on a hunch about Anderson being endangered by an unknown force. She uses him to help unearth the spaceship until they reach the hatch, the effects of the Tommyknockers getting stronger and harsher as more of the ship is exposed. He discovers how the transformed residents (referred to as those who are "becoming") have telepathic abilities and is initially protected from the aliens' effects by a metal plate in his head gained from a skiing collision. He finds more of their technology, most notably the shower stall-crafted stations used to house victims to be used as living batteries. Prompted to kill Bobbi after she's transformed into a Tommyknocker, Gard battles and kills a number of the transformed residents before he boards the ship and activates it, sending it into the sky as he succumbs to his wounds. While it comes at a heavy cost of killing virtually every resident due to the deprivation of the gas and the massive fire that follows its takeoff, Gard's efforts ultimately stop the Tommyknockers from further corrupting other humans.
The Tommyknockers survived the crash in the miniseries adaptation, unlike in the original novel, and serve as more direct antagonists. They utilize a psychic emanation to slowly but surely drain the life energy out of the populace and even capture some of them to trap in pods for such purposes (akin to how victims were used as living batteries in the novel). They are directly confronted when the ship is entered, returning to life to attack Gard. However, they are ultimately defeated and destroyed as Gard takes control of their ship, freeing the townspeople from their control.
In both incarnations, the Tommyknockers are marked by their technological prowess, which is passed onto the residents infected by the gas/psychic emanation. Adding to the various household implementations, such as Anderson's water heater and flying tractor, there are weapons. Among these are a bazooka-like implement made from a culvert that discharges green energy and a repurposed Coca-Cola vending machine (as a flying assailant in the book and a trap bomb in the miniseries). They are also marked by their psychotic temper, which is evident in the residents but downplayed in the miniseries. (This is also noted in how the Tommyknockers violently murdered each other, which caused the crash, and used many of their own kind to power the ship.) Ironically, this mental affliction is what ultimately kept the original Tommyknockers from getting further with their technological advances. Even the residents who have the know-how are still stricken by their psychotic rages. In both cases, they also demonstrate a tendency to overlook simple things, such as using a DC converter on their power supplies.
The Tommyknockers also vary in appearance in both works. In the original novel, they were transparent monstrosities with taloned-feet and gray musculature. This is foreshadowed with details like Anderson's transformed state and the specific details of the ship, such as the ladder with four-foot spaces and semicircular dips in each rung. In the miniseries, however, they have opaque skin and resemble a somewhat more conventional style of hostile alien (likely due to censorship issues that would keep them from showing the organs beneath).
It is also debatable as to what the Tommyknockers truly are when one considers the effect of their gas on others. While multiple alien bodies are found in the spacecraft, the effects on the Havenites hint that the Tommyknockers have been spreading like a virus from species to species on the numerous planets they have visited and ravaged. Anderson herself even somewhat confirms this post-transformation, suggesting that the Tommyknockers would continue this cycle if they were not stopped.
They are also symbolic on a few levels, given their effects on people. Gard, an outspoken activist and critic against nuclear power, takes note of the radiation poisoning that affects the populace and is slowly killing and/or mutating them despite providing them with unlimited energy to use for their lives. On a metafictional level, their effects were also somewhat symbolic of King's then troubles with substance abuse, particularly in how it would lead to bouts of creativity, but would be detrimental to his health.