Sephiroth was born thirty years prior to the beginning events of the original Final Fantasy VII game. His parents, Professor Hojo and Lucrecia, were both assistant scientists to Professor Gast –who was head of Shin-Ra's Science Department and top scientist leading the Jenova Project at that time. Those working on the Jenova project mistook the extra-terrestrial entity code named Jenova as a Cetra - an ancient race of people who'd died out - and used the specimen in an attempt to 'create humans with the power of the Ancients'. They originally believed that this goal would be realized by testing the effects that the Jenova cells had on an unborn child. Professor Hojo offered up his unborn child to the project, and using cells taken from the Jenova specimen, Professor Gast and Hojo injected the alien cells into the fetus within Lucrecia's womb. Lucrecia continued to carry the unborn child to full term while the fetal form merged with the Jenova cells during the pregnancy.
The child created by this experiment was named Sephiroth after birth. He was immediately taken away from Lucrecia after he was born and taken into Shin-Ra's custody. Sephiroth was never told who his father was, or his true mother, only that his mother's name was 'Jenova'. Despite Hojo (his father) being a constant presence in his life, Sephiroth was never aware of that fact and had an extremely low opinion of the man and described him as both 'inexperienced' and a 'walking mass of complexes.'
Shin-Ra raised Sephiroth from birth to be nothing but the perfect soldier who'd fight solely for the betterment of the every-growing electrical company. Trained not only in various battle/sword techniques, which caused Sephiroth to grow extremely powerful both physically and magically, his mind was also trained in war tactics and strategies. Sephiroth rose through the ranks of SOLDIER rapidly due to the enhancements the Jenova cells provided, and by the age of fifteen, he was sent off to battle in the war against Wutai.
It was unclear exactly when Sephiroth was promoted to the rank of General, but knowing Shin-Ra they would have gone into the Wutai war with their guns blazing and throwing their best weapon into the fray with such a high military title. All throughout the war Sephiroth proved himself to be an incredibly strong and efficient member of SOLDIER. He quickly gained worldwide attention for his exploits – especially Shin-Ra's eventual victory over Wutai that Sephiroth helped them achieve. Sephiroth's exploits during the Wutai war also brought in an influx into the Shin-Ra military ranks. Young boys and men from all over the Planet flocked to Midgar (including Cloud Strife) in an attempt to be just like Sephiroth, and joined the military to endeavor to get such reverence and be just like their idol, General Sephiroth.
To Shin-Ra, Sephiroth was considered as President Shinra's 'ace-up-his-sleeve' and was generally only sent out into the field when the President had exhausted all other options. He was sent on only a small number of missions after the Wutai war – most of which consisted of the terrorist group AVALANCHE.
Five years prior to the beginning of the game (Sephiroth was currently age 25); Sephiroth was ordered to take a small team to investigate and inspect the Mako Reactor deep within the Mt Nibel mountains near the town of Nibelheim. One other member of SOLDIER (SOLDIER First Class Zack Fair) accompanied the General along with two troopers (One of which was Cloud Strife who's original hometown was none other than Nibelheim). Upon arrival into Nibelheim Sephiroth began to vaguely recognize the area – or the Shin-Ra mansion to be more specific – in a sense that he felt as though he'd been there before.
Guided up to the reactor by Tifa Lockheart, Sephiroth ordered Cloud Strife to remain outside with their guide while Zack Fair and himself entered into the Mako reactor to investigate the reason behind its malfunction. They discovered several pod-like chambers that contained monstrous creatures within the main reactor. After investigating the pods further, Sephiroth determined that the 'creatures' had once been human and were altered with high dosages of Mako exposure. Shocked by this discovery, Sephiroth soon afterwards found a locked chamber further into the reactor that was labeled 'JENOVA'. Highly disturbed by the fact his mother’s name was written deeply within the reactor and by the possibility of his creation being due to an experiment, Sephiroth left the reactor and headed directly back to the Shin-Ra mansion based in Nibelheim to research the matter further. Locking himself within the Shin-Ra mansion basement library, Sephiroth read over the journals and research material concerning the Jenova Project that was left behind by the Shin-Ra scientists.
The Shin-Ra reports and journals he read through contained their theories of Jenova being a Cetra and their experimental attempts to recreate the Cetra race. With this information, Sephiroth was of the belief that, since he was the son of Jenova, he was the last survivor of the Cetra race, which made him the sole remaining survivor of the Cetra. His reading also contained information regarding the human’s betrayal of the Cetra 2,000 years prior when they left the Cetra alone to defend the planet from a calamity (the calamity actually being Jenova herself). All of this information weighed heavily on the General's already shocked and enraged state. Unable to fully comprehend the information or see it in a rational state, Sephiroth believed it was his right to avenge the Cetra for the human's abandonment 2,000 years earlier, and resolved to take revenge on them for the sake of his 'ancestors'.
Sephiroth attacked and burnt Nibelheim down to the ground after rage consumed his mind. Killing nearly everyone within the village, including Cloud Strife's mother, Sephiroth left the burning town behind as he returned back to the Mako Reactor in order to retrieve his 'mother,' Jenova. His retreat was pursued by some of the few remaining villagers, which included Tifa Lockheart and her father – Cloud Strife and Zack Fair followed along shortly afterwards. Tifa's father was the first to find Sephiroth at the reactor, but the General easily defeated him and, after dying at the hands of Sephiroth, her father's body was left out the front of the reactor's main entrance with the Masamune left beside his body.
Leaving the Masamune behind (the fact that Sephiroth would leave his weapon behind in the first place spoke volumes of how unbalanced and deranged his mind had become), Sephiroth entered into the Mako reactor with the intention of retrieving Jenova from the inner chamber within. Making his way through the pod-room, his advance was halted a second time by the arrival of an enraged Tifa poorly wielding the Masamune. She charged in and attempted to slay Sephiroth with the weapon, but her effort failed when Sephiroth retook his blade during the attack and used the Masamune to cut her down instead.
Continuing on as if nothing had happened, Sephiroth managed to get himself inside of the Jenova chamber but was interrupted moments afterwards as First Class SOLDIER Zack Fair (poor guy keeps getting interrupted D: ) came rushing into the chamber demanding answers. Sephiroth was too far-gone in his insanity to see reason, however, and soon attacked and defeated Zack in battle. Zack's body was thrown out of the inner chamber and back into the pod-room with the force of Sephiroth's attack, and with what he thought was the final distraction gone, Sephiroth attempted yet again to return back to his work with releasing Jenova. It appeared that luck was not on the dear General's side though, and, when he was finally taking a moment to enjoy the reunion with his 'mother', he found himself on the receiving end of an attack with the Buster Sword. Taken by surprise, the large weapon was stabbed through his abdomen by trooper Cloud Strife.
Sephiroth was left for dead as Cloud ran off to tend to Tifa, but unknown to Strife, Sephiroth's enhanced anatomy allowed him to survive the brutal assault despite being greatly wounded. Although he was wounded, Sephiroth's desire to 'free mother' was still strong, but due to injury he was unable to take her entire body. Removing Jenova's head from the containment tube, Sephiroth began exiting the inner chamber with only the thought of taking revenge on the humans for their betrayal 2,000 years earlier, and 'take back' the Planet for the Cetra.
After being instructed by an injured Zack Fair, Cloud ran in to attack and stop the wounded General as Sephiroth re-entered the pod room. Cloud was unable to take Sephiroth by surprise this time, however, and ended up being impaled on the Masamune as Sephiroth stabbed the blade directly through the trooper's chest. Believing to have retaken control of the situation, Sephiroth became surprised by the sheer desperation Cloud showed while impaled on the end of Masamune, and with a dramatic increase of strength, Strife managed to overpower the shocked General. Lifting Sephiroth off the floor by using the blade in his chest as a lever, Cloud managed to pivot his body around hard enough to not only dislodge the Masamune from his chest but to also toss Sephiroth over the edge of the catwalk they stood on and down into the Mako pool beneath the reactor.
Sephiroth's journey didn't end there; however, and after falling into the Mako pool with Masamune and Jenova's head in hand, his injured body traveled through the Lifestream and eventually ended up at the Great Northern Crater. The crater was a wound inflicted on the planet 2,000 years earlier when Jenova first fell from space and down to the planet's surface where most of the Lifestream was called to in an attempt to heal the wound.
After the Nibelheim event, Shin-Ra sealed and buried any records on him and the incident during that time. Shin-Ra even went to such lengths as to rebuilt Nibelheim and populate the town with Shin-Ra employees in order to cover up the disaster. Shin-Ra scientists, lead by Professor Hojo, also took the all the survivors (including Zack Fair and Cloud Strife) into their care and began experimenting on them in an attempt to create clones of Sephiroth. These experiments continued throughout the five years Sephiroth disappeared from Nibelheim, and all throughout those years, nothing was heard of from the late-General and he was presumed dead.
Five years after Sephiroth's original disappearance, former SOLDIER Zack Fair managed to break both himself and a semi-comatose Cloud Strife from the laboratory within the Shin-Ra mansion in Nibelheim. It was around this time that Sephiroth's influence on the planet began to return. Trapped within a large amount crystallized mako in the Northern Crater, Sephiroth had slowly begun rebuilding his greatly wounded body throughout those five years. Unable to move from his mako prison, Sephiroth mentally sent out a call for the Sephiroth clones to awaken and help him to further his ambitions. The Sephiroth clones automatically responded to the call because of the Jenova cells in their bodies and began making their way towards Midgar – the city where Jenova was transferred to after the Nibelheim incident five years prior – including an unsuspecting Cloud Strife.
Shortly after Cloud and AVALANCHE were apprehended in the Shin-Ra building (during their attempted rescue of Aerith), the true Reunion began (Hojo's Reunion theory being that if Jenova cells were separated, they would instinctively strive to gather together again). What remained of Jenova broke out from its containment tube in the Shin-Ra building controlled by Sephiroth's strong will, and the headless body murdered President Shinra while Cloud and company remained locked within the Shin-Ra jail cells. After taking the President's life in a messy bloodbath, the headless form left a trail of blood from the President's room as it traveled all the way down to the jail cells where it unlocked Cloud Strife's jail cell. After giving Cloud an opportunity to escape, Jenova's headless body escaped from the Shin-Ra building and began to wander the world.
While it was believed the headless body of Jenova would be the main focal point for the Reunion, it was Sephiroth's entrapped body up in the Northern Crater that was drawing point for all of those involved in the Reunion.
With the ability to change its appearance, Sephiroth willed Jenova's headless body to take on the image of himself as it wandered the planet's surface. This also included the main body to take on Sephiroth's voice as well as his appearance as it gathered the clones and subtly influenced their actions to further Sephiroth's desires. It was after Jenova's main body escaped Midgar that the Sephiroth clones began to slowly make their way towards the Northern Crater, following their master's call.
Sephiroth continued to guide and manipulate Cloud Strife and the other clones. When Cloud and the other members of AVALANCHE caught up with the fleeing main body of Jenova, it maintained in Sephiroth's image before detaching one of its arms to confront the team. After the defeat of Jenova-BIRTH by Cloud, Sephiroth continued to move Jenova's body around the planet. Leaving subtle clues behind, he allowed AVLANCHE to discover the fact that Sephiroth was attempting to obtain a powerful material known as the Black Materia. This was a dangerous piece of material that would call forth a manifestation of the ultimate destructive magic – Meteor.
Sephiroth managed to obtain the Black Materia after Cloud and AVALANCHE solved the puzzles within the Temple of the Ancients. Manipulating Cloud's mind and body, Sephiroth forced Cloud to hand over the Black Materia to him before leaving the area to continue on with his desires. Sephiroth intended to utilize the Black Materia in order to create a fatal wound to the Planet by summoning Meteor to crash into the planet's surface. In doing so, the Planet would send out large amounts of spiritual energy from the Lifestream in an attempt to heal the wound. While the Planet attempted this, Sephiroth would place himself at the center of the wound to absorb the energy the planet produced. This, along with the gathering of the scattered clones returning to his position, Sephiroth intended to intercept and absorb all of the remaining Jenova cells and Lifestream energy in order to transform himself into a God.
There was only one thing that could stand in his way though, and that was Aerith Gainsborough (the true last survivor of the Cetra race) and the White Materia she possessed. The White Materia contained the spell Holy, which was the only power on the planet that could counter the destructive magic of Meteor. During her attempts to summon the Ultimate White Magic, Sephiroth originally attempted to manipulate Cloud into killing Aerith himself, but Cloud resisted Sephiroth's command for the first time. Instead, Sephiroth ended up using another piece of Jenova in his image to descend down onto the summoning Cetra and impaled Aerith on the end of Masamune. This move killed Aerith almost instantly, and, in a gesture to taunt and further antagonize Cloud Strife's damaged mind, Sephiroth purposefully withdrew the blade from Aerith's body slowly in order to increase Cloud's hatred of him while further destabilizing him for easier manipulation.
Immediately after Aerith's death, Sephiroth told Cloud of how he was only a puppet. His words were interrupted by the piece of Jenova transforming into Jenova-LIFE, and it was only after the battle ended that Cloud managed to hear the rest of Sephiroth's words that there was a possibility that he was nothing but a puppet to be used to further Sephiroth's ambitions.
While Aerith's attempts to summon Holy were successful, Sephiroth's interference of the summoning was also successful to the extent that the powerful defensive spell was being held back within the Planet and prevented its movements by Sephiroth's strong will.
His manipulations of Cloud continued until AVALANCHE reached the Northern Crater where Sephiroth's body was imprisoned. Sephiroth had manipulated Cloud to the degree where Cloud now believed that he was nothing more than a cluster of Jenova cells that had taken on the image of a real person named 'Cloud'. AVALANCHE was forced to fight against Jenova-DEATH at this point in time. It was at this point that Cloud and the others became aware of information from other Sephiroth Clones that the truth behind their purpose was to bring the Black Materia to their master.
After stumbling upon Sephiroth's mako tomb, Sephiroth mentally took control of not only Cloud but also the party member (it changes depending on who the player picks) Cloud picked to safe-guard the Black Materia. Using an illusion on the party member, Sephiroth was able to convince them that Cloud was in need of the Black Materia. Still trapped within Sephiroth's illusions, Cloud and the other party member were unable to hear the warning calls of the others as Cloud was given the Black Materia. Cloud then willingly handed the Black Materia over to Sephiroth inside the mako tomb where his body was still only partially formed. After the materia was handed over, Sephiroth immediately summoned the spell Meteor – an action that backed the Planet into a corner and caused it to released destructive entities it had created known as WEAPONs - before the Northern Crater walls began to give way.
AVALANCHE and the Shin-Ra executives positioned at the Northern Crater at the time were forced to flee the collapsing area as the WEAPON's emerged. The moment they vacated the area Sephiroth erected a powerful energy field around the entire crater to prevent anyone or anything from entering again. While protected within the energy field, Sephiroth focused all of his energy into preparing himself to absorb the spiritual energy into his body after Meteors impact.
The energy field was later penetrated and destroyed by the Sister-Ray (a powerful weapon which uses mako in order to power the device) – which allowed Cloud and the rest of AVALANCHE access to the Northern Crater. The remaining Sephiroth clones also made use of this advantage of the opening and began moving in towards Sephiroth's location. Sephiroth's intentions for the other clones involved sacrificing them into the open mako pools surrounding the area so then he could absorb them into his body. They diffused into the mako, and with the added energies from the Lifestream, Sephiroth was able to gather enough knowledge and spiritual power to begin his transformation from his human form into that of a God.
Despite Cloud and the other AVALANCHE members making their way down into the crater, Sephiroth still had the remaining mass of Jenova's body to prevent any further advances of the group attempting to end his ambitions. The final piece of Jenova formed into Jenova-SINTHESIS and battled the group. Jenova- SINTHESIS was eventually defeated; however, and Cloud and co continued their way down deep into the planet core.
Sephiroth was still in his cocoon stage (Bizarro Sephiroth form) when Cloud and the others arrived, and despite being in this cocoon-like form, he still had enough power to fight back against the aggressors. Cloud and AVALANCHE appeared to be too powerful for this stage though, and Sephiroth was forced to hatch pre-maturely into his final form (Safer Sephiroth/One-Winged Angel) in order for him to absorb the powers of the Lifestream when Meteor hit. Despite all of the powers that came with this form, Cloud and the others were able to defeat Sephiroth before he could realize the fruition of his ambitions.
His body was destroyed during the assault, but before he was absorbed into the Lifestream himself, Sephiroth's spirit made one final mental attack on Cloud's mind. Sephiroth attempted to overcome Cloud's will - like what he had done in the past – and take Cloud's body as his own. His efforts were futile as Cloud mentally fought back against the intrusion and triumphed over Sephiroth's spirit with his ultimate sword technique, Omnislash. Ultimately defeated, Sephiroth's spirit was reclaimed by the Lifestream.
It was also stated in the 'Final Fantasy VII Ultimania Omega Guide' that Hojo's soul was unable to properly diffuse into the Lifestream after his death by AVALANCHEs hands. Instead of following the natural order for those who have died, Hojo allowed his soul to be assimilated into Sephiroth's body so then he could grant his son more power and allow himself the opportunity to witness the ultimate results of his experimentation by being part of it.
Two years following the defeat of Sephiroth and Meteor, a fatal disease called Geostigma began spreading throughout the Planet. Geostigma began inflicting people with extreme bouts of fatigue and open sores on the skin. This disease was a direct result of what happened two years previously, and occurred when Sephiroth and the clones dissolved into the Lifestream before Holy was released. The Holy spell (aided with the power of the contaminated Lifestream) protected the Planet from Meteor's attack, but as it returned back to the planet the Jenova cells, and in turn Sephiroth's cells, began infecting the inhabitants of the Planet.
Despite his death Sephiroth's strong will and spiritual energy managed to create physical manifestations of himself – Kadaj, Loz, and Yazoo – and used the three manifestations to obtain the remaining piece of Jenova (her head) and gather those infected with Geostigma for a new Reunion. This new Reunion's purpose was to bring together enough Jenova cells so then Sephiroth himself could be reborn. After much trial and tribulation, Kadaj eventually gained possession of the remaining piece of Jenova from Rufus Shinra, and after absorbing the contents into his body, his body transformed into Sephiroth himself during a battle with Cloud.
From this moment on Sephiroth's main purpose was divided. He revealed to Cloud that his intent was to use the souls of those that died due to Geostigma in order to use the negative Lifestream to power his will. In doing this, he would take control over the planet and use it as a vessel for him to travel to other words of the universe and thereby continuing the cycle Jenova started over 2,000 years ago.
Yet even despite this Sephiroth had another agenda in mind – to antagonize Cloud and understand how someone like Strife could defeat his power time and time again. It was a concept that had never occurred to the General before, and with the previous defeats by Cloud's hand in the past, Sephiroth couldn't help but focus his frustration and anger over that fact onto Cloud. Pushing aside his ambitions to continue on with Jenova's work, Sephiroth acted on impulse and began attacking Cloud. All throughout the battle between the two, Sephiroth continually taunted Cloud as was obvious to him that physically he was far superior to Cloud. When Cloud showed signs of beginning to tire, Sephiroth's advances turned more violent in an attempt to keep Cloud fighting against him.
When Sephiroth's taunting tactics began involving harming those close to Cloud's heart; however, Sephiroth got more than he bargained for. Cloud managed a renewal of strength, but Sephiroth was not expecting Strife to fight with such determination and conviction. Surprised by this unpredicted turn of events, Sephiroth's ambitions and desires were foiled for a third time as Cloud released his ultimate attack on Sephiroth again – the Omnislash.
Remaining floating up in the air after the attack, one single black and feathered wing appeared from his right shoulder – a remnant of his days as the One-Winged Angel. Disbelieving that he had been defeated again by Cloud, his final words of 'I will never be a memory,' spoke of his strong will and desire to live, and how despite his defeat, he believed that he would return again. Shortly after those words were spoken, Sephiroth's soul was pulled back into the Lifestream along with the remnants of it, Kadaj, Yazoo and Loz.