These lines of verse are from James Montgomery's poem Freedom.
It's the critical moment of the Battle of Sempach (1386), and all will be lost unless a Hero arrives to save the day.
Horrent can also mean expressive of horror. So you can make a horrent cry if, for instance, the top comes off the salt cellar as you're seasoning your soup.
If, say, a gorilla arrives for tea, you might even manage both the hair-standing-on-end and the cry. simultaneously
Thing Probably Not To Do Today: be horrent. This word comes from the Latin horrēns, from horrēre, which means to bristle or shudder.