This growing wish to escape death, known as 'the doom of Men', also made most of the Númenóreans envious of the immortal Elves, or Eldar, who they had come to physically resemble as part of their reward from God (Ilúvatar) for having been their allies.The Eldar sought ever to remind the men of Númenor however, that death was a gift from God to all men, and to lose faith in Ilúvatar would be heretical.
As their power and knowledge had grown throughout the course of that Age, all the Númenóreans had become increasingly preoccupied with the limits placed on their contentmentand eventually their powerby mortality, the purpose of which they began to question; But the fear of death grew ever darker upon them, and they delayed it by all means that they could; and they began to build great houses for their dead, while their wise men laboured unceasingly to discover if they might the secret of recalling life...
('The Window on the West' ~ The Lord of the Rings)These sacrilegious 'black arts', which arose as a consequence of their worship of 'The Dark' and Melkor, marked the final, irrevocable division between the 'King's Men' and the minority known as the 'Faithful' Númenóreans, or the 'Elendili', who kept to their old faith in Ilúvatar.
They were also presumably the earliest religious rituals of those who became known afterwards as Black Númenóreans, along with a disbelief in Ilúvatar.
We're told that the Black Númenóreans emerged from the party of the King's Men.
This was about the same time that the Nazgûl appeared, and as three of those were Númenóreans, we can be sure that Sauron had already ensnared at least some of that race.
The Men of Númenor were settled far and wide on the shores and seaward regions of the Great Lands, but for the most part they fell into evils and follies.