Malantur was the son of Caliondo son of Eärendur, Malantur was a nobleman who lived in Númenor in the later part of its first millennium. He was of very little account in the histories of the time, though events could easily have worked out differently.
King Tar-Aldarion had only one child, a daughter, and according to the old laws of Númenor a daughter could not inherit the Sceptre. Under the laws as they stood, the King's nearest kinsman of male descent would become his Heir. Malantur's grandfather Eärendur was the second son of King Tar-Amandil, and so by the strict interpretation of the law, Malantur would have succeeded Tar-Aldarion to become Númenor's seventh King.
Malantur was relegated to the margins of history when Tar-Aldarion chose to revise the laws, so that both the male and female lines of descent were considered, and his daughter Ancalimë could indeed succeed to become Númenor's first Ruling Queen.