Word for "scary thing that's about to occur"?

I have the feeling I'm forgetting a very simple word. But anyway, is there a word for that? Example:

The [...] that haven't occurred yet are the most scary ones.

Though it doesn't mean “scary thing that's about to occur” I think the word scenarios fit well in your example: the [scenarios] that haven't occurred yet are the most scary ones.

To make it refers to a scary thing you add a describing word like dreadful, frightening and nightmare; the nightmare scenarios that haven't occurred yet are the most scary ones.

The perils that haven't occurred yet are the most scary ones.

noun [MASS NOUN]
1.0 Serious and immediate danger:

1.1 (perils) The risks or difficulties that arise from a particular situation or activity:

There are words you can use to describe scary things, but I do not believe there is a singular word that means "scary thing." That being said, one word you can use is dread:

dread(n): terror or apprehension as to something in the future; great fear. [Dictionary.com]

Dreadful things are the most scary ones.

Another is menacing:

menacing: something that threatens to cause evil, harm, injury, etc.; a threat [Dictionary.com]

Menacing things are the most scary ones.

Another possibility is impending, but you would have to use impending with another word that implies their scary qualities:

Impending awful things are the most scary ones.

I am not sure if this is the word you are looking for, but it's meaning maybe similar.

Foreboding:

noun

1.a prediction; portent.

2.a strong inner feeling or notion of a future misfortune, evil etc.

American Psychological Association (APA):

foreboding. (n.d.). Dictionary.com Unabridged. Retrieved March 25, 2015, from Dictionary.com website: http://dictionary.reference.com/browse/foreboding