Tar-Herunúmen was the (somewhat heretical) Quenya name of the 20th ruler of Númenor, more commonly known by his Adûnaic name Ad. Ar-Adûnakhôr (S/267). His name is a compound of heru “lord” and númen “west”.
Conceptual Development: In the unfinished stories “The Lost Road” and “Notion Club Papers” from the 1930s and 40s, Tolkien used the term ᴹQ. Herunúmen to refer to the Valar as Lords of the West (LR/47, SD/310). Its use as a name of Adûnakhôr did not occur until The Lord of the Rings appendices were written (PM/164, note #11).