Mound of the Elves and Men slain during the Nirnaeth Arnoediad, translated “Hill of Slain” (S/197). It is a combination of haudh “mound”, en “of the” and the plural of dangen “slain”.
Conceptual Development: In the earliest Lost Tales, this name first appeared as G. Cûm a Thegranaithos “Mound of the First Sorrow”, revised to Cûm a Gumlaith of similar meaning (LT1/149). In Silmarillion drafts from the 1930s, the name was changed to N. Cûm-na-Dengin “Mound of Slain” (SM/312, LR/147), then to Amon Dengin “Hill of Slain” (LR/314) and finally Haudh-na-Dengin (LR/312). In The Etymologies, the middle preposition was replaced with the definite article i, Haudh i Ndengin (Ety/KHAG, NDAK) and in Silmarillion revisions from the 1950s-60s it changed to the combined article-preposition ina (WJ/79), then finally to en in Haudh-en-Ndengin (WJ/169).
mound of the slaying; haudh (“mound, grave, tomb”) + en (genitive article) + (n-)dengin (pl. of dangen “slain, slaying”) #We would rather expect it to be Hauth-e-Ndengin, so perhaps it's a dialectal variation of the name; the Etym. gives it as Haudh i Ndengin, which looks more like LR-style S with the second element Ndengin in plural.