[Tiffany]

Originally Posted by Wikipedia

Approximately two thousand years before the main events of the game, Jenova arrived on the Planet, having traveled through space on a meteor. Its landing rendered a giant wound to the Planet that would later come to be known as the "Northern Crater," a blight on its surface that the Planet would channel its Lifestream to in an effort to heal even up to the beginning of the game.[4][5]

After its arrival, Jenova emerged from the Crater and approached the Cetra (also known as "the Ancients"), using her psionic abilities to glean the memories and emotions of the Cetra from their minds, and then instinctually shifting her form in response, assuming the appearances of their deceased loved ones. This allowed her to get close enough to them to infect them with its own cells, which then drove those infected insane and turned them into monsters.[6]

Most of the Cetra were mutated in the time that followed[7], though the survivors attempted to defend the Planet despite their dwindling numbers. They were abandoned in the defense of the Planet by the regular humans (those descended from the Cetra who gave up their Planet-revering lifestyle and subsequently lost their close relationship with the Planet), though a small band of survivors mounted an assault on the Crisis from the Skies and managed to defeat it nonetheless.[8][9] The creature effectively rendered neutralized, it was sealed away in the Northern Crater, where it would eventually be excavated.[10] As a result of Jenova's onslaught, only a few Cetra would survive into the modern era.

Full source can be found here.