The Klingon Civil War of 2292 was a major conflict within the Klingon Empire, which would have devastating long-term effects on the Empire as a whole and was ultimately a major factor in the following year's Khitomer Accords. The war was mostly fought between three factions, including that of Chancellor-designate Gorkon, self-proclaimed Emperor Melkor, and General Chang. (TOS video game: Klingon Academy)
The war had its roots in an attempted coup d'etat by Melkor's older brother Kalnor in 2291, which saw at least one major fleet engagement in orbit of Qo'noS, but was ultimately resolved with Kalnor's death at the hands of Chang, and had little effect on the Empire as a whole. In the following months Chang discovered that Melkor was putting together his own bid for power, and sent his prized student Torlek, son of Ro'vagh on a series of reconnaissance missions to find out more.
The following year, Chancellor Lorak died after a long-term illness, and the High Council designated the incumbent Chief-of-Staff, Gorkon as the new Chancellor. In an unexpected move however, Melkor proclaimed himself the new Emperor, a position which had been unoccupied for centuries, and it soon became clear that he had enough military force to seriously challenge Gorkon. Making matters worse for the new Chancellor, his longtime friend Chang refused to support him due to Gorkon's desire for peace with the United Federation of Planets, and instead formed a third faction.
Though there were theoretically three factions all opposed to each other, for all intents and purposes this was essentially two separate conflicts; one between Gorkon and Melkor, and another between Melkor and Chang. While Chang still intended to oppose Gorkon, he considered the possibility of Melkor becoming Emperor to be a more immediate threat.
Initially the war consisted of a series of major engagements between the Gorkon and Melkor factions, with the smaller Chang faction dedicating itself to destroying Melkor's supply lines. The Chang faction seemingly earned its first major victory by capturing the q'vaQ starbase, a potentially important springboard to taking on Melkor's core territories. However, this rapidly turned into a disaster for the faction, as the starbase had secretly been set to self-destruct after being captured, and the faction suffered substantial personnel losses when the station exploded, with Chang himself barely escaping alive.
After several more weeks of conflict, a turning point came when Chang's forces captured a Romulan diplomat who revealed that in exchange for their support, Melkor would hand them the Tal'Ihnor Gates system, the Empire's main energy production facility. This revelation began to erode Melkor's support, and Chang immediately took a battle fleet to the system to secure it. Unexpectedly however, Melkor had Brigadier K'Mak (ironically the former instructor of Torlek) set off a device which caused the system's sun to go supernova. Chang and his fleet escaped, but the system was utterly obliterated.
Melkor's actions virtually destroyed his support within the Empire, and caused much of his forces to defect to the Chang and Gorkon factions. Subsequently, he challenged Chang to a duel in the nebula that was once the Tal'Ihnor Gates, but when Chang answered the challenge he found Melkor's flagship accompanied by a huge Romulan fleet. Torlek's ship was the only Chang loyalist capable of responding in time, but unable to take on the Romulans alone he enlisted the help of Gorkon and his faction. The Romulans were driven out of the nebula, and Chang was saved, but now owed Gorkon a debt of honor and was forced to allow his faction to be absorbed. While Chang was extremely indignant, this strengthened Gorkon's forces so much that even with Romulan support, Melkor now could not realistically win the war.
Despite this, Melkor made one last gambit. Banking on Chang's fanatical hatred of the Federation, he duped Chang into thinking they had destroyed a Klingon research station, resulting in Chang leading the bulk of the fleet over the Federation-Klingon Neutral Zone in an assault. Torlek was able to get Chang's fleet back to Qo'noS in time to battle Melkor and the Romulans, and during this final battle Melkor's flagship was destroyed, killing the would-be Emperor. After this, his few remaining loyalists and Romulan supporters fled the system, finally ending the war.
In the days that followed, Gorkon ordered that all surviving members of the House of G'Ioch be discommended and put to death. Most of the survivors tried to flee and seek asylum with the Romulans, only to be hunted down and killed by Chang's forces anyway.
Though the war was over, and Gorkon had finally ascended to the Chancellorship, the damage the Empire had suffered was incalculable. Not only had several starbases and shipyards been destroyed, but the loss of the Tal'Ihnor Gates system had crippled the Empire's ability to supply its worlds and ships with much-needed energy, forcing them to mine the moon of Praxis harder than ever. Worse still, the three decade-old alliance with the Romulans was completely shattered by their actions in supporting Melkor, leaving the Empire without any major allies in case of war with the Federation or any other major power.
Chang was appointed as Gorkon's new Chief of Staff, and in turn appointed Torlek as the head of a task force assigned to seek out replacement energy sources for the Empire. In their final discussion, Chang admitted that the infrastructure damage the Empire had suffered would make it easier than ever for Gorkon to finally end the Federation-Klingon Cold War, and implied that he would be taking measures to prevent that from happening. (TOS video game: Klingon Academy)
A year later, the over-mining of Praxis had a devastating result; the moon exploded and caused significant damage to Qo'noS, wiping out its ozone layer. Knowing that the Empire would collapse in at most fifty years without further action, Gorkon finally moved ahead at full pace with his plans to seek peace with the Federation. In turn, this caused Chang to move ahead with his own conspiracy, alongside members of the Federation and Romulan governments. (TOS movie: Star Trek VI: The Undiscovered Country)