The Grey Man is a creature from Irish and Nordic myth with only vague descriptions, vague to the point of being often known as just a ‘presence in the fog’.
The Grey Man included in my monster collection, Creepy, Crawly and Crazy, is inspired by several diverse descriptions, and blended with some D&D rules to make a unique role-playing opportunity. It’s my belief that the best creatures in Dungeons and Dragons aren’t the biggest, baddest, most dangerous ones, they’re the ones that provide an opportunity to drive a story line.
With that in mind, I decided that the Grey Man would be a corrupted Fey; a creature that once roamed the Fey quite happily and then was caught by fiends and corrupted into a creature of pure mischievous evil. Then I decided that they would be related to, and reluctantly work with Green Hags, who are also evil fey.
The Grey Man is quite different from Green Hags in that they like to fool people and have some innate abilities, like belching out fog to obscure vision, but if they work with a coven they can sneak their nasty clawed hands into the covens collection of spells. Green Hags can see the usefulness of working with a Grey Man, but they don’t like that spell-filching one bit!
As a DM I would use the shape-changing abilities of the Hags and Grey Man to manipulate and confuse the players, then use the claws and spells to really wreck them once they figure it out.