Waning of Dúnedain
The waning of the Dúnedain refers to the physical, mental, and spiritual changes experienced by the exiled Men of Númenor after the Downfall of their land at the end of the Second Age. Before the isle of Númenor was destroyed, its people were taller, wiser, and much more long-lived than other Men. After their exile, the Dúnedain who lived in Middle-earth were gradually deprived of these gifts, so that by the time of the War of the Ring, 3,000 years after the Downfall, most of them were on the same level as the Men of Middle-earth. In houses that maintained purity of Númenórean blood (such as that of Aragorn Elessar), these changes were somewhat slowed; Aragorn lived 210 years, far longer than any other Man of that time period.
Broadly speaking, the waning of the Dúnedain could be linked to the decline and ultimate failure of the line of Kings in the exilic realm of Gondor, and the dwindling of the royal house in its sister kingdom of Arnor to a strange, wood-haunting Ranger band. It was also a factor in the gradual depopulation of their lands. In addition to their fall from power and prominence, the Dúnedain suffered mental and spiritual stagnation as well: many of the arts, crafts, knowledge, and technology that the Númenóreans had invented were lost and forgotten. Furthermore, the exiles of Númenor became ignorant and fearful of the Elves, a kindred with which they had been allied in their early days.
The Dúnedain were very conscious of their waning and it was a source of grief to them; they did all that they could to prevent it. Some tried to find ways to prolong their lives, while others urged the Númenóreans to maintain genetic and cultural purity. In the middle part of the Third Age, these beliefs led to the violent Kin-strife when King Valacar of Gondor married a woman from the tribes of the indigenous Northmen. After his son Eldacar ascended to the Throne, many people rebelled against his rule because of his mixed blood. This led to a civil war that threatened to destroy Gondor. However, marriage to the Northmen did not seem to hasten the waning of the Dúnedain—but the catastrophic wars in which Gondor (and its sister kingdom Arnor) were involved certainly did.
Not all of the exiles were concerned with maintaining their status; the Black Númenóreans who lived in the south quickly intermarried with natives of Middle-earth and lost the benefits that they had once possessed as descendants of the people of Númenor.