Here’s an example of a C2 phrase in the English Vocabulary Profile that means:
worry that something very bad will happen or that something very bad has happened
A search for collocates of ‘fear the worst‘ in the NOW corpus:
1 COME 126
2 YET 114
3 BEGAN 59
4 EXPERTS 31
Experts initially feared the worst.
5 STARTED 27
6 MAKES 24
7 REASON 23
8 FANS 23
9 STARTING 19
10 BEGINNING 16