A noun for “collapse, downfall” mentioned in notes for the Markirya poem of the 1950s along with its adjectival form atalantëa (MC/223), likely derived from the root √TALAT. It is also used as one of the names of Númenor: Atalantë “Downfall(en)” (S/281; Let/347).
It seems Tolkien originally based this noun on the past tense of the verb [ᴹQ.] atalta- “collapse, fall in” (Ety/TALÁT); the past form atalante “down-fell” appears in various versions of the ᴹQ. Lament of Atalante from the 1930s and 40s (LR/47, 56; SD/247, 249, 310). This form was also mentioned as an (archaic?) “perfective” past tense atalante “slipped down, fell in ruin” [vs. ordinary past talante “slipped, slid”] of the talat-stem verb talta- in the second version of Tengwesta Qenderinwa from around 1950 (TQ2), though the notes where it appeared were rejected (PE18/88 note #83).
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).