to answer, but not answer the question.
Grendel is the name of the monster in the Beowulf legend. (there's a movie coming out later this year about it). I wonder if she got the inspiration for the name from Grendel. Particularly as Grindelwald was taken down by Dumbledore - given his preoccupation with Inferi, controlling death....

it sounds as though Grindelwald was probably a pre-cursor to Tom Riddle's line of thought really.

but - no - I hadn't heard of them before. (my memory isn't always accurate though....)