A word of sudden and shocking realization

I am preparing a thesis for the rhetorical analysis of a photograph, and I and looking for a word (adjective) that could be used to describe a realization. It should include the meaning of words like ‘shocking’ and ‘sudden’. Such a word would fit well in this sentence:

It was as she noticed the dark connection between her greatest mysteries, the _______ realization set in.

Startling:

• so remarkably different or sudden as to cause momentary shock or alarm.

The Free Dictionary

"Epiphany" is sometimes used to signify sudden moments of clarity. In fact the Oxford Advanced Learner's Dictionary carries the following definition: "A moment of sudden and great revelation or realization."

The dark overtone can be provided by "horror".

I made a slight edit to your sentence:

It was as she noticed the dark connection between her greatest mysteries, the horror set in.

Or without the edit, used as an adjective as you requested:

It was as she noticed the dark connection between her greatest mysteries, the horrific realization set in.