Conceptual Development: In the earliest Lost Tales, this name first appeared as G. Ladwen-na-Dhaideloth “Heath of the Sky-roof” (LT2/287), revised to G. Dor-na-Dhaideloth of similar meaning (LT2A/Dor-na-Dhaideloth, LT2/287). In the Lays of Beleriand from the 1920s, Tolkien revised the meaning of last element of ᴱN. Dor-na-Dhaideloth to “[Land of the] High Plain” (LB/49).
In Silmarillion drafts from the 1930s, it generally appeared as N. Dor-Daideloth (SM/269, LR/250) or Dor-Daedeloth (LR/120, LR/256). At this time the name was usually glossed “Land of Dread”, but this was only accurate as a translation of its shorter variant Dor-Deloth (LR/405, WJ/183), which was also translated as “Loathly Land” in The Etymologies (Ety/DYEL). In one place in this period the name appeared as Dor-na-Daideloth “Land of the Shadow of Dread” (LR/405), a better indication of its full meaning. In The Etymologies from the 1930s, there was a word N. dae “shadow”, which appeared as an element in the variant name N. Daedhelos “Shadow of Fear” (Ety/DAY). These factors are probably the source of Christopher Tolkien’s translation in the Silmarillion Index: “Land of Shadow of Horror” (SI/Dor Daedeloth, SA/dae).
However, in revisions to the Silmarillion map from 1950s, J.R.R. Tolkien translated S. Dor Dae-deloth as “Land of Great Dread” (WJ/183), and later still considered changing the name to S. Dor-na-Daerachas of the same meaning (WJ/187). This indicates that he altered the sense of the element dae from “shadow” to “great”, though exactly when he did so is difficult to pin down.