Vain is an all black creature from Book 6. He is a gift from the Ur-Viles to Thomas Covenant. He only speaks once in the book, and has immense power. For example, when the search enters the home of the Elohim, the creatures are unable to stop Vain from exiting their realm and following Covenant .
Spoiler Alert!!!!!
Vain ends up forcing the Appointed of the Elohim to combine with him to form the new Staff of Law so that Linden Avery can heal the land and destroy the Sunbane.