Be afraid, be very afraid
Ostensibly, a warning that something dangerous is imminent. In reality, this is usually said with comic intent. The thing being warned of is more likely to be mildly unwelcome than actually dangerous; for example, "That fierce librarian was asking about your overdue books - be afraid, be very afraid."
This phrase originated in the 1986 horror film The Fly, written by the Canadian David Cronenberg and starring Jeff Goldblum (as Seth Brundle) and Geena Davis (as Veronica Quaife). The shortened expression 'be very afraid' was already in use in the USA prior to 1986; for example, it was used in the television series All My Children in 1970.
The plot of The Fly has Brundle as a scientist experimenting with teleportation. The scientist is brilliant but eccentric and, naturally, the casting director thought of Goldblum. Just as naturally, before we get far into the film, the experiments begin to go wrong.
Quaife is a reporter working on the teleportation story. When it becomes clear that Brundle is starting to turn into an insect, he pleads with one of the characters "don't be afraid" and Quaife's response is:
"No. Be afraid. Be very afraid."
This was used as a tag line in the film's publicity posters. They also used, "Half man, half insect ... total terror!" and "Something went wrong in the lab today... something very wrong". David Cronenberg, George Langelaan and Charles Edward Pogue were the writers on the film, with the storyline being provided by George Langelaan.
It is the imagery of portentous warnings from the Bible and literature that the film's screenwriters called on to give weight to the line; for example, Shelley's sonnet, Ozymandias:
"My name is Ozymandias, King of Kings:
Look on my works, ye Mighty, and despair!"
There are many similar examples from the Bible, including Isaiah 32:11-11 (King James Version):
"Tremble, ye women that are at ease; be troubled, ye careless ones ..."
Of course, 'be' and 'afraid' are common enough words to have appeared together many times before that, as in Macbeth, 1605:
I will not be afraid of death and bane,
Till Birnam forest come to Dunsinane.
For the use of 'be afraid' as a stand-alone warning, we go back to the Bible. In Romans 13:4-4, in the King James Version, we find:
"For he is the minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is the minister of God, a revenger to execute wrath upon him that doeth evil."
Twenty years has been long enough for the phrase to have been taken into the language, and it is now well enough established to have the shortened form of simply 'be afraid'.