As anyone not entirely allergic to fantasy fiction must know, a nazgûl is one of the nine really extremely nasty sorcerer-kings in JRR Tolkien's The Lord of the Rings. They have a habit of flying about on pterodactyl-like things, have killer breath, are only sort-of alive, are under the influence of that source of immortal cruelty and evil Sauron, and are generally predatory and unwelcome.
Altogether not the people you'd ask to your next barbecue.
So, to nisgul. It's a sadly unusable word because it's bound to get confused with nazgûl, but when you remember that JRR Tolkien was brought up in the Midlands of England, prime nisgul territory, it still makes me smile.
Word Not To Use Today: nisgul. This word is, delightfully, a Midlands English dialect word for the smallest and weakest bird in a brood of chickens.
This Madagascan ant, by the way, is called Tetramorium nazgul.
And I wouldn't want to invite that at my barbecue, either.