This root and similar ones were the basis for “slay” words for much of Tolkien’s life. The root first appeared in the Early Qenya Phonology of the 1920s as ᴱ√dag- with numerous etymological variants, including ᴱQ. taila/ᴱN. dail “axe (blade)” < dagla, ᴱQ. tanga-/ᴱT. danga- “to beat” < tang-, and ᴱQ. nahta-/ᴱN. dag- “to slay” (PE14/65-66); these last two words for “slay” appeared regularly in Tolkien’s writings thereafter. The root reappeared as ᴹ√NDAK “slay” in The Etymologies of the 1930s with similar Noldorin derivatives (Ety/NDAK), apparently a strengthened form of ᴹ√DAK “slay” (EtyAC/DAK).
The root was given as ᴹ√NDAG “slay” in the Quenya Verbal System (QVS) from 1948 (PE22/102, 115), but unglossed ᴹ√NDAK appeared in the same document (PE22/112), and verb ᴹQ. nak- was sometimes glossed “kill” as well (PE22/120). The root appeared as √NDAK “hew, slay” in the Outline of Phonology from the 1950s (OP2: PE19/91) and again as √NDAK “hew” in Late Notes on Verb Structure (LVS) from 1969 (LVS: PE22/156). This introduction of the sense “hew” in the 1950s and 60s may be a partial restoration of the senses of the 1920s root ᴱ√dag-, which meant more that just “slay”. If so, √NDAK may be compared with √MAK which had a similar variety of meanings (“cleave, sword, slay, fight, forge”); see that entry for details.
This root first appeared as ᴹ√RIS “slash, rip” in The Etymologies of the 1930s with the derivative N. risto “rend, rip” (Ety/RIS¹). Tolkien then created a new entry for ᴹ√RIS without deleting the prior entry, with derivatives like ᴹQ. rista/N. rhest “a cut” and ᴹQ. rista-/N. rhista- “cut” (Ety/RIS²); this seems to reflect a conceptual shift of “slash, rip” >> “cut”. Indeed, the root √RIS reappeared in Tolkien’s later writings with the gloss “cut” (PE17/87). Christopher Tolkien gave this root the gloss “cleave” in the Silmarillion Appendix (SA/ris).