Long before the infinite souls of the underworld invaded Midgar, humans have been fighting against Soul Army fyears.
The Soul Army call their leader supreme commander the "Immortal". He was once a sage king of humans then corrupted by some unknown force.
The sage king didn't have an heir when he was turned the "Immortal". That led to an long lasting civil war in the kingdom of humans.
When the humans are busy fighting their war. The Immortal manages to create a rift on the boundary that separates the human world the underworld. Dead souls quickly pour out of the rift flood the human world. The boundary has been pushed deep the world of humans.
The humans have finally grasped the seriousness. The warring factions have formed an expedition army. Their quest was to find the legendary Death Blade that possesses the power to destroy the Immortal. Apart from the Death Blade, the humans still need to find the shards of the Seven Sins Gems which are scattered all around the continent during the war between the Immortal the Kesh. According to the legend, only the combined power of the gem the blade may destroy the Immortal once fall.
With the help from the Gods, the expedition army manages to travel through the Soul Sea marches the heart of the underworld. Finally, the humans will confront the Immortal's Seven Sins Army.