According to Wikipedia, Grim is also one of the many old names for Woden, Northern European King of the gods. It is believed that the name Grim attached to several ancient earthworks in England is to honour Woden. Similar earthworks are sometimes named for the devil, make of that what you will.
The earthwork I am particularly interested in today is Grim’s Dyke or Grim’s Ditch, which runs very close to where I am currently living, in Hatch End. It runs for a length of about 5km from Pinner Green towards Harrow Weald and was built in Roman Britain times, probably by the local Catuvellauni tribe, as a defense against the Latin invaders.