The root √BAL had to do with “power”, and in particular divine power. Its best known derivatives are Vala and Valië, the greatest of the Ainur and divine rulers of Arda. In Tolkien’s earliest writings, he describe the Vala as gods, but later he treated them more like “archangels” under the divine patronage of the one god Eru.
The earliest forms of this root were VALA in the Qenya Lexicon of the 1910s (QL/99) and GWAL in the contemporaneous Gnomish Lexicon (GL/44), so very likely the true early form of this root was ✱ᴱ√WALA. The Quenya form of this root VALA was unglossed, but the Gnomish form GWAL had the glosses “fortune, happiness”. At this early stage the root had additional derivatives like ᴱQ. valin “happy” and G. gwaltha “fortunate, happy” (QL/99; GL/44).
By the time of The Etymologies of the 1930s, these “happy” derivatives had fallen away, and the new form of this root was ᴹ√BAL with new Noldorin forms like N. Balan “Power, God” (Ety/BAL). The root √BAL reappeared in the 1950s and 60s with various glosses like “powerful, mighty” or “have power” (PE17/48, 154). One relatively late development was the transfer of the word S. Balrog “mighty demon” to this root (PE17/48; S/31); in earlier conceptions this word was tied to “torture” roots like ᴹ√ÑGWAL “torment” or ᴱ√MBALA “hurt, pain” (Ety/ÑGWAL; QL/63).