Jenova (ジェノバ, Jenoba?) is an extraterrestrial life-form
Once Jenova lands upon a planet it will destroy every form of life it finds. Jenova can absorb its prey's memories and form, hiding as their loved ones to destroy them.
Once Jenova has destroyed a planet it uses it as a vessel to travel the cosmos to another planet.
Approximately two thousand years ago, an alien creature landed on Gaia, having traveled through space on a meteor.
Jenova can very well make it's way into the game, not hard, and won't conflict with Eorzea lore at all seeing as how it's an extraterrestrial being