The word "tak" itself appears to mean, simply, "big" and/or "powerful" in the Language of the Dead spoken by Tak.
Unlike most forces of evil in the Stephen King Universe, Tak is incorporeal and seems to be completely unable to form a body on its own, at least on Earth. In Desperation, Tak possesses the bodies of living things, which quickly wear out due to the stresses Tak's energies apparently place on living tissue. In the parallel world Earth of The Regulators, Tak partially takes over the mind of an autistic boy named Seth Garin, but the boy's consciousness remains within the host body, unlike the minds of those possessed by Tak in Desperation.
In both cases, the creature manifests itself in Earth's dimension via an entry point (called an ini) in the bottom of a very deep well in the China Pit, a once abandoned but recently re-opened mine shaft in the town of Desperation, Nevada, located near the Desatoya Mountains. In Desperation, Tak's physical body is actually located in another dimension, where Tak is apparently trapped with only a tiny opening in the ini connecting it to the outside universe. However, in The Regulators, the entirety of Tak's form consists of an intangible mist with bright red specks throughout, although this mist too originated from the China Pit. In both novels, Tak seems unable to move around in Earth's dimension as a free-roaming, disembodied spirit for very long without a corporeal host.
Powers and Abilities
Tak's powers vary within the context of the individual books in which it is featured.
In Desperation, Tak's abilities include mental control of animals and insects and the transmission of its essence into any host body it chooses within in a certain range. Tak first accomplishes this by forming makeshift arms made of a smoky, viscous substance. Each arm ends in three points, two to enter a victim's nostrils and one to enter the mouth. The points are tipped with small "holes like eyes" through which Tak's essences passes into its victims. After this, Tak only needs to connect the mouth of the current victim with that of its next intended host. The less physically robust the victim, the less time that the host body will last; Tak's energies took nearly a full week to wear down the body of the already physically massive and healthy sheriff, Collie Entragian, while a sickened woman, Ellen Carver, lasted only a few hours. Though it primarily uses human hosts, Tak can take over large animals such as timberwolves or golden eagles, though these are not remotely as strong as human hosts and will "burn out" in a matter of hours. It is noted in the book that if Tak were to inhabit anything too small, the host will explode instantly. The body of the host/victim physically grows in size and strength; even the relatively frail Ellen Carver's body was grown to a size nearly as large as that of her husband's after being infused by Tak. The souls of those beings, sentient or otherwise, who are invaded by Tak are seemingly forever displaced; the moment that Tak leaves a host body, the host falls lifeless to the ground regardless of the damage that may or may not have been done to it. When the host body dies without Tak transmitting itself to another, all the creatures under its control at that time are killed. Tak can only inhabit one host at a time, but it can take control of other humans through the use of can tahs ("little gods" in the Language of the Dead) in the form of small stone animals. However, the degree of control is lesser and the affected human will die a gruesome death if separated from the can tahs.
The can tak ("big god") itself - that is, the combination of the host body and Tak's essence - has superhuman strength, feels no pain, and has the ability of supernatural sight that allows it to "see" in an undetermined radius all around it, even through solid objects. On one occasion, Tak was able to see a name plaque inside the Carvers' RV even though he hadn't stepped foot inside or looked through any of the windows. On another occasion, while looking straight down in the front seat of his police cruiser, Tak brought up a loaded shotgun with his other hand and kept it locked onto the prisoner in the rear seat, Johnny Marinville, who unsuccessfully tried to avoid it. In addition to the supernatural sight enjoyed by his host body, Tak can also see through the eyes of any animal under his control; he once even located a target by literally "seeing" what a flying bat's sonar could echolocate. Furthermore, he can see through many pairs of eyes in this manner at exactly the same time and form a mental picture from the multiple inputs.
Tak also has the mysterious ability to "infect" over time inanimate objects that lay in the vicinity of the ini, turning them into can tahs. These artifacts quickly drive to murderous insanity almost any human beings that pick them up. Finally, at close proximity to the ini, Tak can partially override the movements of living beings; in this way was it able to force its first victim, Cary Ripton, to open his mouth against his will and take Tak's essence into his body. Tak attempted to possess Johnny Marinville in the same way, but Johnny wore a motorcycle helmet with a face plate to deflect Tak's advances.
In The Regulators, Tak's abilities are even more powerful. While in the mind of Seth Garin, Tak has the ability to alter reality itself. The nearby landscape (though, inexplicably, not the humans trapped in the area) begins to take on the cartoonish aspects of Seth's childhood fantasies, albeit with Tak's murderous proclivities and appetite for chaos. At one point, Tak is able seemingly to remove Poplar Street from the world altogether, placing it into a pocket dimension of his own design (c.f. Jerome Bixby's "It's a Good Life"). Although the images Tak creates are actually not there and are, in fact, just visions, the visions can cause physical damage to living beings, such as the "Regulators," a group of aliens and cowboys Tak creates from Seth's imagination. These imaginary Regulators shoot and kill many people on Poplar Street.
Unlike its Desperation counterpart, this Tak seemingly cannot inhabit anyone's body but Seth's. Apparently only Seth's body is strong enough to contain Tak, apparently due to the boy's nascent mental powers (which, it is implied, allow Tak to grow powerful enough to warp reality in the first place). On the one occasion that it tries to change hosts, Tak overloads and blows up a human's head from the inside mere moments after entering the new body.
The Tak of Desperation could not repair the physical defects in his host bodies; in fact, his magical energies sped up the various defects in such host bodies to such an extent that even normally harmless defects soon burned the body out - and forced Tak to constantly look out for replacements.
In Cary Ripton's case, a case of cancer that would have killed him in months went into overdrive once Tak possessed the body, and Tak was forced to dispose of it after only about 24 hours. Tak's next host, Brad Josephson, had a leaky heart valve, a congenital birth defect which might have persisted harmlessly for years unmolested but which was rapidly worsened by the mystic energies involved and which forced Tak to leave that body behind. Tak's strongest host, Collie Entragian, was in almost perfect physical condition but was easily sunburned; nevertheless, he lasted almost an entire week before it began literally falling to pieces, seemingly unrelated to his skin sensitivity. Finally, Tak's fourth and last host Ellen Carver had nothing more than an otherwise "harmless" yeast infection, but the condition was enough for Tak's energies to cause that body to bleed out after less than twelve hours. The novel does not make it clear just how long a perfectly healthy host body would last.
The Tak of The Regulators, on the other hand, was almost powerless without a host body and could only use exceptional host bodies such as that of Seth Garin's. Other, non-exceptional bodies blew up from the inside out in seconds if Tak tried to possess them. This Tak was also completely unable to keep Seth's body running (heartbeat, respiration, circulation, et al.) without the boy's essence present. For this reason and this reason alone, Tak did not expel Seth's consciousness once it co-opted Seth's body. Moreover, despite its incredible appetite for cruelty, gore, and death, Tak is nevertheless thoroughly repulsed by the act of defecation and always exited Seth Garin's body whenever it occurred. Seth and his aunt Audrey Wyler use this weakness to defeat Tak.
Defeat and Re-imprisonment
In both novels, Tak's underestimation of its enemies - both human and Divine - proves in the long run to lead to its downfall.
In Desperation, Tak is "outsmarted" by God Himself; most of the central characters who found themselves trapped in Desperation each had his or her role to play in either distracting Tak or confronting it outright. The characters, particularly David Carver, theorize God led them to Desperation to defeat Tak. At the end of this novel, Tak finds itself to have been manipulated all along, bereft of host bodies, and re-imprisoned in the China Pit mine by a disastrous explosion which buried the Rattlesnake No. 1 Shaft and caved in the surrounding area to such an extent that it appears unlikely any mining expeditions would ever again be launched in the vicinity.
In The Regulators, Tak finds itself outsmarted by the extraordinary intelligence of autistic Seth Garin, who leads Cammie Reed to kill him to prevent Tak from further using Seth's powers. Tak later swears revenge on the survivors, but without a host body - and specifically, a special host body with similar powers to Seth's - it is all but powerless.
Connections to The Dark Tower and other Stephen King Books
Tak is never mentioned by name in any other Stephen King book, although some phrases of Tak's Language of the Dead make their way into various Stephen King books and short stories regarding the The Dark Tower. Unlike many other forces of evil in the Stephen King Universe, Tak seemingly does not answer to anyone or anything else, including the Crimson King (nor does it make any reference to him). One edition of The Stephen King Desk Calendar refers to Tak as "Tak the Outsider," further emphasizing Tak's lack of allegience to any other force - and its imprisonment in another dimension. The Desperation Tak is aware of the existence of God in Keystone Earth but claims He is not present in Desperation - though the other characters prove it wrong.
In The Little Sisters of Eluria, The Little Sisters speak the Language of the Dead, as Tak does. The Little Sisters make their camp in Eluria near the Desatoya Mountains, which in the world of Desperation and The Regulators is the location of Desperation, Nevada.
In It, one of the novel's heroes describes the true form of the main antagonist, Pennywise, as "deadlights." While It's deadlights are white, Tak's red miasma and lights in The Regulators take a similar form. Tak and It do share some characteristics but are clearly different types of beings.