I guess my issue here, is how does a planet loving race create a "planet killer" spell? It just seems like their should be more to this than it was just made and sealed. Especially when its explained in the game that Black Materia needs more power than a regular person can give and requires large amounts of mako energy to cast. I just can't see how this spell would be practical for them. Was it perhaps created by the first humans who branched away from the Cetra lifestyle to build normal settlements, yet still retained some of the Cetra's power before it would be lost to humanity by time? Was it a rogue Cetra or a group of them that turned from the teachings and created some fanatical way of becoming one with the planet by wiping all life on it away and just return to the Lifestream? Was it created to counter some evil force?

I personally liked the Jenova theory, that it originated with her, but looking at the mural in the Temple of Ancients, it does look like this was used by the Cetra before Jenova, hell its possible that Jenova arrived on the Planet while riding the original Meteor spell. So it's possible that its the Cetra's own fault Jenova came to Gaia.