Legend had it that Praxis was a gift given to the Klingons from their gods. The legend came from the era when the Klingons first traveled into space and was the first world they visited away from their own planet. (ST reference: Federation: The First 150 Years)
When the Klingons first went into space, they regarded Praxis as gift from the Klingon gods. It was the first world visited by the Klingons, who found it contained a rich mineral supply. For many years, the Klingons used the molten core of Praxis to provide the Empire with energy. The Klingons were the first people to test such a procedure and their work was duplicated by the Bajorans, who tapped their moon of Jeraddo to provide a similar source of energy for Bajor. (DS9 - Millennium novel: The Fall of Terok Nor; ST reference: Federation: The First 150 Years; DS9 episode: "Progress")
In 2293, over-mining and insufficient safety precautions led to the destruction of Praxis in a massive explosion. The shockwave from the explosion damaged Qo'noS's ozone layer, hit the USS Excelsior, and the remains of the moon formed an asteroid belt around Qo'noS which threatened all life on the planet. Klingon scientists estimated Qo'noS would not be able to support life just thirty-four years after the disaster, and the Federation predicted the fall of the Klingon Empire within fifty years as a result. (TOS movie & novelization: The Undiscovered Country; TOS novel: Sarek; TOS comic: "The Order of Things")
The devastation also opened a political rift within the Empire, half of the High Council favored a typical Klingon response of striking out to take whatever the Empire needed to recover from the disaster. However the other half, led by the Chancellor Gorkon, suggested requesting aid from the Federation to help the Empire repair the damage. The Council finally decided in favor of Gorkon's plans and opened peace talks with the Federation. (TOS comics: "Against Their Nature", "Losses"; TOS movie: The Undiscovered Country)
Some people believed that information learned from the explosion of Praxis was used in the so-called Omega Device. Specifically, it was thought that the device may well be used for a similar effect: a subspace shockwave. (Last Unicorn RPG module: Planetary Adventures)
In the alternate reality created by Nero's temporal incursion, Praxis had exploded by the year 2259. It was later revealed that Section 31, under orders from Starfleet Command CO Admiral Alexander Marcus, authorized two missions; the first mission was to just destroy the facilities. However, the squad's ship was detected and Lt. John Harrison was captured. In the second mission, "John Harrison" used a personal transporter to beam in alone. There, Harrison took out the Klingon guards, planted demolition charges, and then beamed down to the Ketha Province on Qo'noS. Praxis was then destroyed. (TOS - Khan comic: "Issue 4")
- It is not known if there was a Lt. "John Harrison", let alone a previous mission to Praxis. Given Section 31's nature, it is possible that they used the mission in order to coerce Khan unknowingly into joining them.
When Kirk and Uhura took a security team to the Klingon homeworld in pursuit of the renegade John Harrison, Praxis was seen from orbit, shattered. This caused severe devastation on the planet, laying waste to much of Ketha Province. (TOS movie: Star Trek Into Darkness)
In an alternate reality where a war ensued between the Klingon Empire and Federation over the Federation's refusal to share the technology of the Genesis Device, the device was deployed against the empire on the Klingon moon of Praxis.
Having reached a stalemate in the five year war, the Federation felt that deployment of the device on Praxis would act as a demonstration of force and lead to a possible Klingon surrender. Although the moon had a population of about 500,000, the Federation thought that since most either lived underground or in colonies protected by shields and domes, the inhabitants would be spared any ill-effects from the Genesis wave as it re-formed the moon's surface. The Katai, a Klingon ship in the possession of the Federation, made the journey to the Qo'noS system under the command of Admiral Thelin and deployed the Genesis Device.
Out of the 500,000 inhabitants on Praxis, only about 2,000 survived the terraforming of the moon as most died due to shields overloading or residing in settlements that proved to be too close to the surface. The attack led to an immediate surrender of the empire to the Federation.
It was later discovered during peace negotiations on Khitomer that deployment of the Genesis Device had prevented the moon from potentially exploding within a year's time. (Star Trek: Myriad Universes novel: The Chimes at Midnight)
In the mirror universe, starting in 2289, Praxis was the site of a secret facility used to construct special Birds-of-Prey which could fire torpedoes while cloaked. To power the production, and the cloaking field that kept it hidden from Terran spy arrays the energy production on Praxis was tripled.
Four years later, in 2293, Praxis exploded, causing massive ecological damage to Qo'noS: toxic elements from Praxis' crust began to break down the atmosphere and contaminate water supplies, threatening the end to higher-order lifeforms within fifty years. Additionally seventy percent of the population were inflicted with deadly xenocerium poisoning.
However, unlike in the primary universe, the incident did not inspire peace between the Klingons and Humans, despite an offer from Emperor Spock to provide medical and scientific assistance. Instead, the Klingons formed an alliance with the Cardassians and conquered the Terran Republic, using the fleet of modified Birds-of-Prey begun at Praxis. (TOS - Mirror Universe novel: The Sorrows of Empire)
- It seems probable the ecological effects of the Praxis explosion in the mirror universe will have been the same as in the primary universe. It is also possible the cause - over taxation of the moon for the secret production of modified Birds-of-Prey - was the same, or similar, in both universes.
In one permutation of the mirror universe, Praxis was destroyed by Klingon terrorists in 2293, in an attempt to create chaos among the Terran colonists residing on the surface of Qo'noS. (Decipher RPG module: Through a Glass, Darkly)