From book 1:

When the shattered fragments of the Black Moon collapsed onto planet Earth, splitting continents apart and flooding them with raging oceans, Darkness descended onto our world. The eruptions of hostile energy from behind the Edge distorted the laws of the physical world, creating new life forms which now threaten to take over the planet. The last remaining vestiges of humankind, too desperate for their own survival, have nothing to offer against their advance.

So it’s time to activate Incarnation: the last instruction issued by Project Stellar. In a situation when there’re not enough living beings to protect planet Earth, the dead fighters will have to rejoin the defenders’ ranks.