Final Fantasy VII
Reunion is the terminology used to refer to the actions that are undertaken by various people in the Final Fantasy VII universe. The basis of this is a congregation of everyone affected, aiming to reach a certain point, so that everyone is gathered together.
The cause of this phenomena is down to experiments that were undertaken on certain individuals involving Jenova cells. The Reunion is, in effect, these cells inside the hosts attempting to rejoin with Jenova. It is a mass calling to where she currently is.
Those affected in Final Fantasy VII were completely oblivious to the world around them and this can be seen as the individuals dressed in black capes only had one goal in mind, getting to the location of the Reunion. The Reunion itself was due to be located at the Northern Crater where Sephiroth was waiting with Jenova.
Many of these people were marked with tattoos indicating their experiment number, most of which were injected by Hojo. It is also to be noted that many of these individuals never make it to the meeting point, and instead die on route returning to the lifestream in the process.