Punctuation is a beautiful thing.
And once more with feeling "Dwimmerlaik" is a Rohir word, but not a Rohir word for Nazgul. It means "magic-work" - in other words, something unnatural, sorcerous and unclean. In The Two Towers, Tolkien goes out of his way to mention that the Rohir have no dealings with magic. So a Rohir describing something as a "magic work" is a nasty insult.
In theory, a Rohir scholar might refer to Wights and Wraiths and those things in Cirith Ungol and the Army of the Dead and some other strange creatures as "Dwimmerlaik."
That being said, I'm about 80% certain that yes, the Dwimmerlaik as envisaged by GW will be a Rohir inspired figure. I am also certain that we'll be told something about how the King of Men in question will be a Northman king of the upper Anduin Vales or something. (Tolkien nerd comment: that's not where the Rohir originally come from
)
All we know (from Tolkien) about the Nazgul is that Khamul was an Easterling, the Witch King ruled Angmar in the Third Age, and three of the Nazgul were Numenorean lords. It's not much of a stretch to assume that the Witch King was one of said Numenorean lords. And...that's it.
One idea that's occurred to me before is this:
There are Seven Dwarven Houses, and Seven Dwarven Rings. Coincidence? Would that imply that there were nine tribes or Houses of Men? We know of three houses of the Edain (The House of Beor, the House of Haleth and the House of Hador. Haleth and Hador are almost certainly the ancestors of the Dunlendings and Northmen respectively. Assumably this means the Dunedain are drawn from the House of Beor). So are there six other tribes of Wild Men in the East and South (and North)? Note that three Edain became Nazgul. Three houses of the Edain, three Rings of Power. Six Tribes of Men who were distinctive enough in the Second Age for Sauron to choose a lord of each tribe to enslave.
But I digress.
Gavin