Banshees are loyal to their households like brownies. Some say they are beautiful and others say they're hideous.  All the same, they are terrifying to see. The song of the banshee is beautiful and creepy. Banshees are known to carry items of someone who is about to die. They foretell death.

"Some believe that preventing a banshee from mourning or refusing to listen to its song will delay death, but this seems unlikely as, in most cases, a banshee is not the cause of death but merely its herald."
(direct quote from Arthur Spiderwick's Field Guide)

* photo courtesy of Arthur Spiderwick's Field Guide