# messengers will go out from before me in ships to terrorize a secure Cush

Messengers will bring the news of Egypt's destruction to Cush, who will be frightened by the news.

# to terrorize a secure Cush

"in order to frighten Cush, who now feels safe from danger"

# there will be anguish among them on the day of Egypt's doom

"the people of Cush will also suffer when I punish Egypt"

# behold!

"indeed!" This adds emphasis to what follows. 

# It is coming

"It" refers to the "great pain" or "great sorrow" that Cush will have when they are punished along with Egypt.