The reason behind the Third Secret being horrendous is that it fits in with the descriptions of the coming Apocalypse.
This entails a devastation on all fronts, famine, pestilence and war, however it goes deeper than that in so far as the spiritual implications of chaos and ultimate subjugation under a perceived 'saviour' of mankind in the darkest moments.
Your senses are grossly overwhelmed by these events to the point of desperation and you become swept up in the deception so as to actively contribute. You are lost at this point and will fall into the narrative as if your life depends upon it... guess what!
Quoting: Anonymous Coward 78345073 Re:12:13-17:
And when the dragon saw that he was cast unto the earth,
he persecuted
the woman which brought forth the man child.
And to
the woman were given two wings of a great eagle,
that she might fly into the wilderness,
into her place,
where she is nourished for a time,
and times,
and half a time,
from the face of the serpent.
And the serpent cast out of his mouth water as a flood after the woman,
that he might cause her to be carried away of the flood.
And the earth helped the woman,
and the earth opened her mouth,
and swallowed up the flood which the dragon cast out of his mouth.
And the dragon was wroth with the woman,
and went to make war with the remnant of her seed,
which keep the commandments of God,
and have the testimony of Jesus Christ.