( rowena gives him a look as if to say don't be an idiot even though she knows that he knows very little about the supernatural )

It's a spirit.

( emphasis on the word. but she does at least give him some more context )

A caoineag, our banshee, is told in lore as foretelling death. Most legends say they are disembodied souls, so cutting off her head wouldn't be an option. The deaths she's foretelling are ours -- if you can hear her scream it is your death that comes. My spell is preventing her scream from affecting us, even if we can still hear it.

( a brief pause as she looks at him to see if he's following so far )

There are spells that will weaken and banish her but I don't have the ingredients to cast it. A blade of gold would kill it if you could get close enough.

( and rowena could trap it to make that easier )