Geez, I love this guy! Now without further ado, the backstory of Seph...
During his days as a general, Sephiroth was the epitomy of SOLDIER. He possesed extreme levels of swordsmanship, and a fantastic use of materia. But his unnatural prowess turned out to be just that, "unnatural" as he found out 5 years ago in an expedition to investigate a malfunctioning Mako reactor in the mountains of Nibelheim.
Along with Zack, a fellow member of SOLDIER, and Cloud, a Shinra guard, Sephiroth entered the reactor that contained creatures that had been injected with the Jenova entity. By finding out that Jenova's name was also his mother's, Sephiroth launched a search into his past history in the Shinra Mansion Basement. It contained documents and reports done by Professer Gast, who had led the "Jenova Project" previously.
Acoording to th reports, an investigation was launched in order to figure out how the ancient race, Cetra, was linked with the Promised Land, a place where one can find supreme happiness and where it is believed that it is brimming with raw Mako energy that can be tapped for immediate use. Shinra, the maker of infamous Mako reactors, found this too tempting to resist, even if it was a legend.
2000 years ago, a lifeform was found that was believed to be of Cetra blood and was named Jenova, and Shinra's Jenova Project began. Professor Gast was appointed chief scientist to head the project. Shinra ordered Gast to produce an army of soldiers exhibiting powers of the Cetra. So many were placed in the pods within the reactors, and infused with a combo of Jenova Cells and mako energy. Later, these specimens would be known as SOLDIER.
Sephiroth was Gast's ultimate success story from the Jenova Project. Dumbfounded by the fact that he was articficially produced, he would later go insane and burn down Nibelheim.
It was Cloud who struck the fatal blow and fell the mighty Sephiroth. Shinra initiated a coverup on the whole incident, rebuilding the town and hiring people to work as townspeople. Jenova was shipped back to Shinra headquarters where Hojo continued monitoring the Cetra's status.
Armed with the power of Jenova, he would bide his time and later manifest himself. He would take his mother's body and release his rage against the world. However, Aeris, a true descendant of the Cetra, stood in his way. Aeris would meet her death at the hands of Sephiroth, But also revealed his true motives.He wanted to go to the promise land, so there would be enough life energy to summon meteor. If Meteor struck, Planet would gather all its Lifestream energy to heal the wound that would have been caused. Sephiroth would feed off of that energy, in the hopes of becoming an almighty being. But in the end, it was Cloud who would vanquish Sephiroth once and for all.
Although Sephiroth's actions and manipulations were inexcusable, he is not totally at fault. He was living an ultimate lie and never did discover his true past. All he ever found was Gast's documents, and that information was false. Jenova was not a Cetra, but an alien being that the Cetra referred to as the original "crisis from the sky". It was Jenova who infected the Cetra race with a deadly virus which virtually wiped out the entire race. Jenova was not Sephiroth's mother, nor did Gast "produce" him. Instead, Sephiroth was the child of Hojo and Lucrecia, Vincent's secret love. Hojo, in an attempt to find truth in his reunion theory, gave up Spehiroth while was still in the womb.
Sephiroth was a normal child, but was forced to live another man's dream. Living a lie that would soon drive him insane, you can't help but feel sorry for him.
