Elendili or Elf-friends are a faction of Númenóreans who advocated continued friendship with the Elves. They were also called the Faithful for their continued devotion and obedience to the Valar. Their most noted leader was Elendil, Lord of Andúnië, who later founded the kingdoms of Arnor and Gondor in Middle-earth.
By the close of the Second Age Númenóreans had become split between the Elendili and the King's Men — a faction centered around the King that sought to assert Númenórean authority and self-determination. With Númenor reaching the apex of its might, the King's Men eventually espoused open defiance of the Valar. This split would eventually precipitate the Downfall of Númenor.
The Elendili, however, not only preserved their ancient friendship with the Elves, they also regarded the burgeoning arrogance of the King's Men as blasphemy. But the King's Men became more powerful, and by the end of the Second Age had begun to persecute the Elendili as rebels and dissidents. Fearing their growing strength, the King's men secured the Faithful's deportation from their strongholds in the western regions, notably around the city Andúnië.
The Elendili enjoyed a brief respite when Elf-friend Tar-Palantir assumed kingship and began to turn Númenor back to the ways of the Faithful. But after Tar-Panatir died, his nephew Ar-Pharazôn usurped the throne and the Elendili were more vigorously oppressed, this time with the help of the Dark Lord Sauron, who had established an evil cult on the island to corrupt and eventually destroy Númenorean society.
As Ar-Pharazôn led his grand armada to Aman to challenge the Ban of the Valar, Elendil and the remaining Elf-friends were warned by the divine powers to leave Númenor forever. They were thus spared the downfall of Númenor when, as punishment for an attempt to defy the Ban, the Valar sank the island kingdom into the sea.
The Elendili, under the leadership of Elendil and his sons, eventually made their way to refuge in Middle-Earth where they were welcomed by the Elves. There they established the Dúnedain kingdoms of Arnor and Gondor.