Sephiroth was a top ranking SOLDIER officer, the most powerful and skilled fighter there ever was, many people admired him. Sephiroth's final mission for Shinra was to investigate a Mako reactor in the Nibelheim mountains. When in Nibelheim, Sephiroth reveals that he never had a proper family, his mother, whose name was Jenova, died after giving birth to him, Sephiroth doesn't ever mention anything about a father, but he does say he has no home town. They get to the reactor and inside Sephiroth discovers capsules containing monsters, he says that they were once human, but have been exposed to a large amount of Mako energy, so they mutated. He also says that Shinra SOLDIERs are given small doses of Mako energy to give them more power. Aswell as the capsules there was a chamber at the top with 'Jenova' written above it, Sephiroth realises that it is the same name as his mother's, he starts to think. He wasn't a standard SOLDIER, he was the most powerful and the strongest SOLDIER in the Shinra army, this made him wonder if he was 'created' in the same way as the monsters in the capsules (since Jenova was kept in a chamber next to them), was he an experiment? Sephiroth's files were secret, kept locked away somewhere. Somehow Sephiroth knows of the hidden library in the Shinra mansion (back in Nibelheim) and decides to go there and find out his true origins

Sephiroth disappears for days in the Shinra Mansion library, reading everything he can find that might be connected with who he really is. He finds out two major things, about the Ancients and the Jenova Project. He learns that he was created through the Jenova Project, and that Jenova was his mother. Here's a bit more on that project...

It began when scientists found an organism, 2000 years old which they identified as being one of the Ancients (Cetra). It was named, Jenova. The Cetra were an Ancient race of people who lived on the planet before humans did. They had special powers and could communicate with the planet. When they died they would go to the promised land, a place which Shinra decided would be full of Mako energy, so of course they wanted to find it. They appointed three scientists to work on the Jenova project, where they would recreate the Cetra using the cells of Jenova. Shinra wanted the recreated Cetra to lead them to the promised land. And it was through this project that Sephiroth was created.

Sephiroth also read about the Ancients, he learned that the planet once belonged to the Cetra. A Crisis struck the planet, it put the lives of everyone and ultimately the entire planet in danger. Some of the Cetra fled from the danger, leaving the rest to die whilst saving the planet. The cowardly Cetra were the ancestors of humanity. Since Sephiroth was the son of Jenova, who was a Cetra, he believed that he was the rightful heir to the planet, and as the humans were traitors (since their ancestors were the Cetra who ran away from the crisis) they should be punished for what they did and he must return the planet to it's rightful owners, him and his mother, Jenova.

Upon discovering what he believed to be his true identity, Sephiroth, in a fit of rage, anger, and hatred towards humanity, burned down Nibelheim and slaughtered everyone there (except Cloud, Tifa and Zack) he then proceeded to the Mako reactor to be reunited with his mother. Inside the reactor both Tifa and Zack tried to stop him and failed, then Cloud stabs him in the back, naturally Sephiroth skewers Cloud with his masamune, but unfortunately that wasn't the end, as Cloud somehow manages to chuck Sephiroth into the Mako pit at the bottom of the reactor. After this event, it was reported that Sephiroth had died. Shinra decided to move Jenova to their headquarters to carry on their experiments.

Then five years later, someone attacked the Shinra headquarters and stole Jenova from her chamber, this person killed everyone there including president Shinra, who was found with a long sword sticking out of his back... Cloud identifies this person to be Sephiroth due to the masamune sword, as Sephiroth is the only one in the world who can handle it. But, Sephiroth was meant to be dead?!

But actually Sephiroth wasn't exactly dead (heh). Mako energy is really the lifestream of the planet, the force which keeps it alive (although Shinra don't know this, or maybe they do and don't particularly care) and after Sephiroth was thrown into the Mako pit, he was carried by the lifestream and ended up cocooned in a capsule of Mako in the northern crater. Here he continued his plans, with the power of his mind (and jenova cells most likely) he controlled various Sephiroth clones (or some kinds of manifestations of himself) to search for the promised land, the place where the Cetra went to become one with the planet. And he also searched for a way to punish humanity for being 'traitors'.

Sephiroth travels around the world, and goes to the Temple of the Ancients, here he absorbs the knowledge of the Ancients and creates his master plan. He would use something called the black materia, with this, he would summon Meteor, the most powerful force known. When Meteor hits the earth, it will injure the planet so badly that an immense amount of lifestream (mako energy) would be sent to the wound by the planet to try and heal itself. Sephiroth would wait in the wound and merge with the lifestream and become one with the planet. This would make him and Jenova rulers of the planet and also destroy everything, thus punishing the humans.

The only thing that can stop Meteor is Holy, contained in the white materia this equally powerful force would save the planet from destruction. Aeris is the only one who knows this and goes to pray for Holy. Sephiroth kills Aeris while she is in the middle of praying and continues with his plans. Unfortunately Aeris had managed to do enough. After Sephiroth was killed by Cloud and his friends, the planet was saved by Aeris' prayer and good triumphed again.

 

Cloud never did make SOLDIER, but he did become good friends with a member named Zack. With his friend, Cloud has his first mission with Sephiroth. This occurs when the reactor at Nibelheim must be checked. By truck, Cloud, Zack, Sephiroth and two other soldiers (one driving) make their way to the village. Sephiroth and Zack know it's Cloud's hometown, but Cloud doesn't particularly want to return since he hasn't made SOLDIER yet, and feels he will be a disappointment to his mother and to Tifa and her father. This is why Cloud is so ardent about keeping his helmet on during his time in Nibelheim, and the voyage to the reactor. Sephiroth also returns to his "hometown" in this voyage, and senses there is something familiar about the place. However, he doesn't divulge more information.

After spending some time at his mother's house catching up with her, and admitting he hasn't made SOLDIER, Cloud is consoled by her, and sets out with the rest of the team. Tifa, now being trained in the martial arts, ends up being their guide to navigate the Nibelheim mountains. Cloud still hides behind his helmet, not wanting her to know he's returned.

Once everything is checked, the group returns to the town. However, Sephiroth secludes himself in the Nibelheim mansion's library, reading Professor Hojo's notes about how he was created and was supposedly the first man-made ancient. Probably influenced a lot by JENOVA dwelling in his mind, Sephiroth snaps and begins wreaking havoc on the town: He burns it down, and runs toward the reactor in the mountains. In this fire, Cloud's mother is killed.

Enraged, Cloud chases after Sephiroth. Both Tifa and Zack have also followed the general. On his way into the reactor, Sephiroth stabs Tifa's father, leaving his sword embedded in his corpse. Tifa enters at that moment, pulls the sword from her father, and runs at Sephiroth. He grabs her and slashes her across the chest, entering a secluded part of the reactor where JENOVA is held, and cuts off her head. Zack then enters and lays Tifa aside, promptly chasing Sephiroth into the back room. Soon after, Cloud arrives, and leans over Tifa, taking care of her. This is when Zack flies out the back room, onto a reactor module and, badly injured, tells Cloud he must stop Sephiroth. Aghast that his childhood hero would do such dishonorable things, Cloud races up the stairs and confronts Sephiroth. For the first time, Cloud throws off his helmet and openly chastises the general. Annoyed, Sephiroth stabs Cloud through the chest, but is surprised when Cloud retaliates by grabbing the blade and lifting Sephiroth off the ground, throwing him into the mako below. Cloud then collapses.

For five years, he and Zack are held in ShinRa laboratories, where scientists attempt to make them clones of Sephiroth through JENOVA and mako treatments. Cloud has an adverse reaction to the mako, and his mind becomes confused. He is unable to function on his own when Zack breaks out, traveling through the nearby forest in attempts at escape. All the way, he carries Cloud with him. However, before he can escape from the dangerous grounds around the facility, a group of soldiers attack Zack, who is shot and killed. Cloud then assumes partially his identity.