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[Final Fantasy VII] What is Sephiroth?

Just finished watching Final Fantasy VII. I have played VIII but not VII. I just know, is Sephiroth some type of mystical being. What is he?
gpt
Sephiroth in the FF7 movie is dead. Or quasi-dead, existing only as a force of will in the Lifestream. The three Remnants of Sephiroth, Kadaj, Yazoo, and Loz, are what physically remains of him, pseudo-clones generated by the power of the Lifestream. Their goal is to find the remains of a being called Jenova, which has the power to fully resurrect Sephiroth.

What Sephiroth was before he was dead is a different matter. What the main focus of FF7 is regards an ancient race of beings called the Cetra (or just referred to as ancients), which are believed to be extinct, who had the power to control the Lifestream, which is made up of energy called Mako. Sephiroth (as well as two others, Genesis and Angeal) were attempts by the Shinra Corporation to resurrect the Cetra by using the genetic material of a long-dead being called Jenova. Jenova was mistakenly believed to be a Cetra, but was actually an ancient alien that almost destroyed the planet two thousand years prior. While the subjects of Project Jenova were not the Cetra-Human hybrids that Shinra wanted, they turned out to be incredible beings of immense strength, speed and fortitude. Lacking imagination, Shinra made them the top tier of their super soldier program; SOLDIER, and used similar process to create other super soldiers, although without the use of Jenova cells (actually this part is unclear, the only thing used for sure is Mako Energy).


***Spoilers for FF7 below:***
***
The main "crisis" in the FF7 universe actually happens years before the original game. While investigating the reports of monsters spawning from a Mako reactor, Sephiroth and Zack Fair (both 1st Class Soldiers), as well as a no-name Shinra guard called Cloud, discover the location of Jenova. This leads Sephiroth to discover his "true" nature (as a Cetra-Human hybrid and experiment test subject, but really his actual nature goes undiscovered in his lifetime) and goes insane, destroying the nearby town before fighting Zack deep in the reactor's core, and being killed when Cloud pushed him into the Lifestream beneath the reactor.