[William Cloud Hicklin]

Well, Tolkien used the term, as he did "fell beast," but in both cases without capitals and as a mere descriptive phrase. A realm, which was in woodlands, rather like a beast, which was fell (deadly, fearsome). It was the screenwriters who somehow converted them into proper names.

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"Greatest" king is interesting, since it implies there were others. This makes little sense in the developed post-LR history, but did make some more sense when TH was written, and was kinda-sorta-maybe set in First Age Beleriand, and Thingol (whom the Elvenking more or less was) could be considered the "greatest" (given that this was post Beren and Luthien and thus after Fingolfin and Finrod were dead). Remember that at this early stage in the legendarium, the people of Beleriand or Ilkorins were not Eldar; nor were the Wood-elves even after TH was published until their later promotion from Avari to Nandor.