You will hear Sephiroth mention 'the reunion' a few times in the game. So, what is the reunion? Well, mad scientist Hojo who was working on the Jenova project thought up this reunion theory, it was that if Jenova's cells were scattered they would find each other again and become one, the place where they would meet would be the promised land. It was called the reunion for the obvious reason that the cells would reunite with each other. Hojo injected test subjects with Jenova cells to see if his theory would work. These test subjects are the black-cloaked figures you see at certain places (temple of the ancients, Nibelheim, and the northern crater). They could be related to the Sephiroth copies which he controls because some of them manage to mention Sephiroth it's as if they are 'his followers'. Something which I thought of whilst playing the game was that the citizens of Nibelheim who were killed when Sephiroth burned it down were taken away and injected with Jenova cells and turned into the black-cloacked figures that you see when you go there, I also read an agreeing theory at 'Sephiroth no Oukoku' a long time ago, so it might be right