Apocalypse is the name given in later years to the overlapping of the physical and element planes.
Note to self: explain here how direct conveyance drew the planes together.
With the overlapping of the planes and the awakening of the Guardians, the path from birth to death is unguarded. Dark things, nightmares, creep out of the darkest forests of the spirit world and prey upon dreamers and those traversing life and death as never before.