As their power and knowledge had grown throughout the course of the Second Age, the Númenóreans had become increasingly preoccupied with the limits placed on their happiness - and eventually their power - by 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.. ('Akallabêth' ~ The Silmarillion)
This growing wish to escape death, known as 'the doom of Men', made most of the Númenoreans envious of the immortal Eldar, who they had come to physically resemble. The Eldar sought ever to remind the men of Númenór however, that death was a gift of the One God, Ilúvatar, to all men, and the will of Ilúvatar could not be gainsaid.
Nevertheless, after S.A 2221, when Tar-Ancalimon became King of Númenor;
- ..the people of Númenor became divided. On the one hand was the greater party, and they were called the King's Men, and they grew proud and were estranged from the Valar and the Eldar. ('Akallabêth' ~ The Silmarillion)
The 'King's Men', ultimately became vulnerable to the corruption of Sauron, who, having arrived in Númenór;
- ..naturally had the One Ring, and so very soon dominated the minds and wills of most of the Númenóreans. (Letters ~ No. 211)
Eventually, in Númenor's last years, its powerful and elderly King Ar-Pharazon, who had become "frightened of old age" (Letters ~ No.156), was persuaded by Sauron that Ilúvatar was a lie invented by the Valar, and seduced him;
- ..back to the worship of the Dark, and of Melkor the Lord thereof, at first in secret, but ere long openly and in the face of his people. ('Akallabêth' ~ The Silmarillion)
Within Númenor, the majority quickly followed suit, and this worship quickly passed across the ocean to most of Númenor's colonies in Middle-earth;
- 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. Many became enamoured of the Darkness and the black arts.. ('The Window on the West' ~ The Lord of the Rings)
These 'black arts' and 'follies' were presumably the earliest culture traits of those who became known afterward as Black Númenóreans;
- ..for in the days of the sojourn of Sauron in that land the hearts of well nigh all its people had been turned towards darkness. Therefore many of those who sailed east in that time and made fortresses and dwellings upon the coasts were already bent to his will.. ('Of the Rings of Power and the Third Age' ~ The Silmarillion)
This worship of Melkor and the Dark marked the final, irrevocable division between it's adherents and the 'Faithful' Númenóreans, the 'Elendili', who kept to their old faith in Ilúvatar, but these were a small minority;
For many centuries after the Downfall, some descendants of the 'King's Men' held onto what became the most northerly and famous of their realms-in-exile, Umbar, although
- ..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. ('Of the Rings of Power and the Third Age' ~ The Silmarillion)
Most of those few Númenóreans who had never envied the Eldar, and had always remained true to their belief in Ilúvatar, also survived the destruction of their homeland, and they established their own realms-in-exile north of Umbar, where previously had come
- ..only the Faithful of Númenor, and many therefore of the folk of the coastlands in that region were in whole or in part akin to the Elf-friends and the people of Elendil.. ('Of the Rings of Power and the Third Age' ~ The Silmarillion)
- ..the Black Númenóreans; for they established their dwellings in Middle-earth during the years of Sauron's domination, and they worshipped him, being enamoured of evil knowledge. ('The Black Gate Opens' ~ The Lord of the Rings)
The Black Númenóreans held a similiarly low opinion of 'The Faithful' and their descendants, as
- ..they inherited without lessening their hatred of Gondor. ('Appendix A' ~ The Lord of the Rings).
Two early Black Númenórean lords are named from the time of the late Second Age: Herumor and Fuinur. Like all Black Númenóreans and 'King's Men' before them, Herumor and Fuinur desired power over men of other, lesser races, and they "rose to (great) power amongst the Haradrim", the peoples neighbouring Umbar. Their fate is unknown, but they likely shared Sauron's defeat at the hands of the Last Alliance of Elves and Men.
The Black Númenórean style of governing was no doubt tyrannical, but may also have involved a tradition of duumviracy, at least in Umbar, whose lords are usually paired when mentioned; Herumor/Fuinur for example, were probably rulers of Umbar, as much later Angamaite/Sangahyando were. Whatever political system was in place, however, the Black Númenóreans did not govern effectively;
- ..some were given over wholly to idleness and ease, and some fought amongst themselves, until they became conquered in their weakness by the wild men. ('The Window on the West' ~ The Lord of the Rings)
The triumph of the Last Alliance marked the decline of the Black Númenórean race and the end of their racial superiority;
- After the fall of Sauron their race swiftly dwindled or became merged with the Men of Middle-earth.. ('Appendix A' ~ The Lord of the Rings).
Nevertheless, a Black Númenórean elite survived at least in Umbar for over a thousand years after Númenor's fall, maintaining much influence in Haradwaith. As late as 1015 T.A., for example, even after being exiled from their homeland for nearly a century;
- ..the Men of Harad, led by the lords that had been driven from Umbar, came up with great power against that stronghold.. (from 'Appendix A' of 'The Lord of the Rings').
The Black Númenóreans did not use Westron, but probably retained their old tongue Adûnaic, speaking a dialect of it. (In The Notion Club Papers, part of Sauron Defeated, Arundel Lowdham cited two descendants of classical Adûnaic. One of these must have been Westron, the other the tongue of the Black Númenóreans (Black Adûnaic?).
The Black Númenóreans are absent from recorded history after their defeat by Ciryaher in T.A 1050., but a population of sorts must have survived somewhere at least until the end of the Third Age, as The Mouth of Sauron, who mocked the army of King Elessar in front of the Morannon was described both as a Black Númenórean and "Renegade", which is presumably the term used by the Free Peoples of that time to describe all folk of similiar ancestry.
In one of his letters, Tolkien wrote that Queen Berúthiel, wife of Gondor's King Tarannon Falastur was a Black Númenórean, from a Black Númenórean realm he describes as "the inland city" somewhere south of Umbar. This was a loveless union, and was presumaby a political accommodation: that such arrangements were possible implies the existence at that time of more Gondor-friendly Black Númenóreans than the much later Mouth Of Sauron.
Three of the Ringwraiths can be considered among the first and most powerful Black Númenóreans, even though their origin predates Númenor's fall by about 1000 years: they served Sauron, being enslaved to his will, having become so because of their lust for power or knowledge..