Banshee: a female spirit of Irish legend which visits a family to announce the impending death of one of its members by screaming. Alternatively, the banshee shows up after the fact as a mourner.
Which is to say that "banshee" is not appropriately used as a proxy for some vague extremity. The following usages are ill-informed:
--Pushing out that baby hurt like a banshee.
--The dripping two-year-old ran down the hall naked as a banshee.
--The basement was darker than six feet up a banshee's bum.
Someone desiring rhetorical employment for the term "banshee" is probably looking for the cliche "screaming like a banshee." But that's a cliche, so why use it? The banshee has lost nearly all rhetorical force between becoming a cliche in her own right and consequently being forced into other nonsensical tropes. Can she be revived in correctly applied metaphor? I believe so. In the right context, one of the following might be of use (please note, some uses are ironic):
--informative as a banshee (alternatively: sympathetic as a banshee)
--subtle as a banshee
--upbeat as a banshee
--Law-oriented as a banshee
I'm not Irish, people, but let's get this right.