Quenya 

atalantë

downfall, overthrow, especially as name [atalantë] of the [downfallen] land of númenor

atalantë noun "downfall, overthrow, especially as name [Atalantë] of the [downfallen] land of Númenor" (DAT/DANT, TALÁT, Akallabêth, SD:247, 310; also LR:47, VT45:26). Variant atalantië "Downfall", said to be a normal noun-formation in Quenya (Letters:347, footnote). From the common noun atalantë "collapse, downfall" is derived the adj. atalantëa "ruinous, downfallen", pl. atalantië in Markirya (changed to sg. atalantëa this change does not make immediate sense, since the adjective undoubtedly modifies a plural noun, but Tolkien does not always let adjectives agree in number).