This page contains information regarding Star Trek: Lower Decks, and thus may contain spoilers.
Landru was built powerful enough to manage the affairs of an entire planetary population. It established totalitarian rule over Beta III for about 6,000 years, managing the affairs of each individual and striving to meet the ambitious goals its builder set it. Its subjects were oblivious to the fact that they were ruled by a computer. Since Landru was hidden behind a solid wall, it is likely that its builder intended this; even millennia later, some of the citizens of Beta III believed the Landru who ruled their world was the same one who saved it so many centuries ago, and none realized it was a machine. The passing years of peaceful rule had inculcated in the people a kind of reflexive worship of Landru. This could be seen even in members of the underground who sought freedom and actively opposed Landru's will.
To meet Landru's goals, his machine was given the ability to control the attitude and conduct of individuals, through a process called absorption. Once absorbed, a living being's individuality and free will were largely subordinated to the instructions and ideas supplied telepathically by Landru. Within the parameters of Landru's guidelines, referred to as the Directives, the individual had some free will. Absorbed individuals were referred to collectively as the Body. Landru viewed the Body as analogous to the body of a living being; it referred collectively to the memory of the body, and to outsiders as "infection".
To enforce its will, Landru maintained an army of lawgivers. These brown-robed individuals were under extremely deep control; they lacked all volition except what Landru supplied. When Landru was forced to devote most of its power to solving a paradox, it withdrew its direct influence from its lawgivers, causing them to panic. Lawgivers carried staffs with which they could absorb individuals who were not part of the Body, or in extremis, kill. Landru preferred to absorb its enemies, killing only when it believed it had no choice.
In 2167, the Archon visited Beta III and encountered Landru. The specific sequence of events remains unclear, but from information supplied by the underground, it can be concluded that some event caused Landru to attack the Archon, possibly with the same heat beams it later used against the USS Enterprise. As the Archon's orbit decayed, her crew fled to the surface, where many were absorbed, and many others killed. The fate of the Archon remained a mystery for a hundred years.
In 2267, the Enterprise arrived at Beta III, seeking to learn the fate of the Archon. Lieutenants Sulu and O'Neil were dispatched to the surface and were quickly discovered and absorbed, forcing Captain James T. Kirk to organize a larger landing party. This landing party also quickly ran into trouble and was captured by Landru. Several members were absorbed before assistance from Marplon, a member of the underground, helped Kirk and Spock escape. They confronted Landru in the Hall of Audiences, confirming what they had earlier guessed: that Landru was a machine, and not a living being.
Landru threatened Kirk and Spock with obliteration, likening them to a strong infection. It believed their deaths, and the deaths of all who had seen them or knew of their existence, were necessary to cleanse the memory of the Body. Spock realized that it might be possible to reach it by questioning the value of its leadership. In asking what Landru had done to do justice to the full potential of every individual of the Body, Kirk forced the machine to confront a truth it had avoided for 6,000 years: by reserving creativity to itself, it was destroying the Body – it had become the evil against which it was charged to protect the Body. It expended so much computing power attempting to resolve this paradox that it began to withdraw its influence from even its lawgivers. But it failed. Throughout the encounter, it had repeatedly asserted its identity as Landru, but in the end it made a final plea to a man six millennia dead, imploring its creator for help. And then, in a shower of sparks and a cloud of smoke, it ceased to operate, freeing both the Body from their thralldom and the Enterprise from the threat of destruction. (TOS: "The Return of the Archons")
At some point following Kirk's mission to the planet and prior to 2380, Landru reassumed control over the planet's populace and was subsequently confronted by Captain Carol Freeman and Commander Jack Ransom of the California-class USS Cerritos, who once again removed it from power by reiterating Captain Kirk's original message to the population. Ensigns Beckett Mariner and Brad Boimler also took part in liberating the planet on a unofficial level, distributing art and other creative hobby supplies to the children of the planet before being beamed onboard the Cerritos.
The encounter left a impression on Captain Freeman, who expressed dismay Starfleet's lack of diligence about Beta III, and how despite it having previously been discovered, it had not received attention in the century since Kirk's visit which allowed Landru to once again resume power over the population. (LD: "No Small Parts")
In her reference book BFI TV Classics - Star Trek (p. 47), Ina Rae Hark proposed that the concepts of Landru and his control over Beta III's population were inspired by "contemporary fears of communist collectivism or blissed-out youth drug cultures."
Landru was referenced in an early title for the episode "The Return of the Archons" – namely, "Landru's Paradise". Similarly, the character was also referenced in some foreign titles of the episode (such as in its German and Japanese names). (Star Trek Concordance, Citadel ed., p. 23)
The manifestation of Landru was played by Charles Macaulay. Regarding this performance, director Joseph Pevney said of Macauley, "He loved doing it." (The Star Trek Interview Book, p. 192) Macauley's apparently projected appearances as Landru in "The Return of the Archons" were accomplished with use of double exposures. (The Star Trek Compendium, 3rd ed., p. 55)
In the book BFI TV Classics - Star Trek (p. 44), the technological version of Landru was cited as one of several unsupervised computers in TOS that each enslave humanoids for what it considered to be for their own good (other such artificial intelligences being Vaal in "The Apple" and the androids of "I, Mudd"). Similarly, the book Star Trek: The Original Series 365 (p. 117) cited the computerized Landru as the first of multiple computers on which Kirk induced self-destruction.
In the ninth and tenth issues of IDW Publishing's Star Trek: Ongoing comic series set in the alternate reality, it's revealed that Landru was actually a prototype artificial intelligence of Federation origin created in 2167 by the head of Starfleet's Advanced Research Division Cornelius Landru to assist in colony development. But Landru's true intentions were more sinister as he was attempting a means of population control. On Beta III, he conducted experiments on the colonists, programming them to his whim to create a utopia where he would be seen as a god. The crew of the USS Archon attempted to stop him, but the ship was destroyed and its crashed remains were built into a temple by the descendants of the crew's survivors. As civilization on Beta III began anew, Landru hid his technology from the colonists and watched over his experiment, reveling as it unfolded. Landru eventually died at some point before 2258, but he lived on through his machine. Using transwarp beaming, the crew of the USS Enterprise ripped the heart out of the computer that was controlling the minds of the inhabitants freeing the people and ending Landru's reign. After the mission, Admiral Pike was admonished by an unseen superior for not keeping a tighter leash on his protégé Kirk after exposing the secret experiment that lasted for decades.