§ Mornennyn >> Morannon (WR:112)
§ Ennyn Dûr >> Morennyn >> Mornennyn (WR:113)
§ Kirith Naglath ’Cleft of the Teeth’ (WR:137)
These are preceding forms of the Black Gate of Mordor, in The Lord of the Rings it is called Morannon (LotRIV, ch.3).
The elements involved here in several combinations are N. morn ’black’, cognate to Q. morna or N. †môr ’black’, cogante to Q. more < mori (MOR-, VT45:35; Let:347) (or else just a prefix mor- directly from the stem); and N. annon, pl. ennyn ’great gate’ (AD-). Many European languages put ’gate’ or ’door’ into the plural (a gate is naturally formed out of two halves), e.g. Slavic vrata, vorota is always used in this plural form. It seems that Tolkien imagined the same to happen in Noldorin/Sindarin at some point in time.
The form Ennyn Dûr is peculiar, since the latter part of it is the adjective dûr ’dark, sombre’ (DOƷ-, DÔ-) which does not agree in number with the pluralized ennyn. The usage of ennyn as singular despite being grammatical plural seems unlikely, since Nelig Morn and Naglath Morn occur at the same time (see 3.11). It may be just a slip (Nelig Morn is also immediately changed, Naglath Morn deleted) or perhaps dûr < *doʒrē is here meant to be the noun *’blackness, darkness’ and is put into genitival position.
Kirith Naglath is from cirith ’cleft, ravine, defile’ < kir- ’cut, cleave’ (RC:334-335, Silm.index, cf. KIR-) and naglath collective pl. ’teeth’, for which see 3.11.