The Ayleids, also known as the Heartland High Elves or Wild Elves, were the first race to establish an empire in Tamriel, and had ruled modern-day Cyrodiil for countless years dating back to before recorded history. Ayleidoon, their language, is similar to Aldmeris, the ancient language of the Aldmer. The Ayleids were the original founders of the Imperial City, which they called the "Temple of the Ancestors". However, their empire collapsed in the early First Era. While it's believed that the Ayleids remain in the wilds of every province on Tamriel, sightings have been few. There have been no known sightings in the Third or Fourth Eras.
Much like how the ruins of the lost Dwemer dot Hammerfell, Skyrim, and Morrowind, the Ayleids have left ruins scattered around Cyrodiil and other regions which were once under the dominion of their Empire. These underground cities can prove deadly for adventurers, hiding such dangers as traps, monsters, undead, and criminals. Their cities were amazing feats of architecture ranging from small settlements to gigantic, labyrinth-like metropolises shaped out of whitish rock. They also seem to have had a firm grasp of the magical arts, as many of their ruins contain stones filled with magicka essences harvested from the sky. They also developed what came to be called the school of Alteration, as they were skilled in magics such as shapeshifting and levitation. As for their appearance, like all elves, they were most likely thin and lean with pointed ears and angular facial features. Their complexion was lighter than that of the Dunmer, but they were darker than Altmer. They preserved many of the ancient runes and Dawn Era magics of the Ehlnofey that other elves had forgotten. The prophecies of the moth-eyed foretold that Ayleidic hubris would "bear bitter fruit", a warning that many, apparently, failed to heed. It would be their undoing.
For more information, see the main lore article.