Herumor is mentioned only once, in the passage cited below;
..because of the power of Gil-galad these renegades, lords both mighty and evil, for the most part took up their abodes in the southlands far away; yet two there were, Herumor and Fuinur, who rose to power amongst the Haradrim, a great and cruel people that dwelt in the wide lands south of Mordor beyond the mouths of Anduin. (from "Of the Rings of Power and The Third Age" in The Silmarillion).
In "The New Shadow" in The History of Middle Earth series, Herumor is also the name of the leader of a growing evil cult in Gondor during the Fourth Age about one hundred years into the reign of Eldarion, the son of Aragorn. This Herumor could be the same as the Númenórean ruler of the Haradrim, as there is some speculation that he became either one of the ringwraiths or the Mouth of Sauron.
In the defunct and somewhat obscure Middle-earth Role Playing game from the 1980s, Herumor is given an extended history. Tolkien, however, had nothing to do with writing this history. Fuinor is then his older brother.