Zilart are an ancient race that existed before the Hume, Elvaan, Mithra, Galka, and Tarutaru. The ruins of Fei'Yin, Ro'Maeve, as well as many of the other ancient ruins around Vana'diel are relics of their lost civilization.
SPOILER WARNING: Details about the Final Fantasy XI in-game storyline follow.
They look similar to Humes and are exceedingly intelligent. Their original home was the Celestial Capital Al'Taieu, where the fifth mothercrystal was located. They were endowed with the gift of telepathy (or "whisper of the soul"), thus enabling them to share their thoughts with each other. But in time, the darkness of the Emptiness began to taint some of them, and their telepathy was silenced. These were the Kuluu, who became outcasts due to the darkness.
The Zilartian Prince Eald'narche came into contact with the crystal, and beheld a vision of paradise. This vision was shared through the whisper of the soul and the Zilart sought to restore the original mothercrystal and open the gates to Paradise. They built the floating island in the sky -- Tu'lia -- to facilitate this end. But the terrestrial avatars, Kuluu, and the Dawn Maidens opposed them; opening the gates to paradise would unmake Vana'diel.
The conflict came to a crescendo when they sabotaged the Crystal Line, resulting in the Meltdown, and the avatar Bahamut destroyed Al'Taieu. The city and the mothercrystal sank to the Sea of Shu'Meyo; the city's defenses shifted the sinking city into the Lumoria dimension. After the Meltdown, the five races and many others came to dominate Vana'diel during the next ten thousand years.
But the Zilart -- and a few Kuluu -- survived in stasis within Delkfutt's Tower, until they were awakened by the Shadow Lord's pact with the Dark Divinity. Many of them now run the Duchy of Jeuno's government, including the Archduke of Jeuno, Kam'lanaut, his older brother, Eald'narche, and Duke Vicarious Esha'ntarl.