According to the Silmarillion, the nineteenth King of Númenor called himself Adûnakhôr, a name that should have only been used as a title of the Valar. He foresook the use of elven-tongues and would not allow their use in his hearing. The attitude against the Valar was so great by this time that it even affected this naming of the kings. In fear of Valar, however, the name was instead more properly written as Herunúmen in the Scroll of Kings.
However, in RotK Appendix A, the text says it is the twentieth king who takes the name of Ar-Adûnakhôr. The list when counted, however, shows him as 19th.
Not only is the name of Adûnakhôr (Lord of the West) supposed to only be used for the Valar, it is best used for Manwë, the Elder King of the Valar. Adûnakhôr punished those that used the Elven-tongues openly and persecuted the Faithful Númenoreans. In his time, the Eldar stopped coming to Númenor.
Adûnakhôr was the son of Tar-Calmacil. Ar-Adûnakhôr was succeeded by Ar-Zimrathôn. (These are shown earlier in the list, after Appendix A tells of Elros.)Ar-Adûnakhôr became king S.A.2899.