Goodnight Robicheaux is an assumed name. He has no real last name; the one he uses is borrowed from a sweet little old lady he first met on one of his early trips into the human world. The name he was born with is Ascha Oresa. He is a very high ranking demon, born of and in hell as the nine-hundred ninety-ninth child of the Devil himself.
In short: Goodnight is an Antichrist but not the Antichrist. He hasn’t been born yet, so Goodnight is the baby of the family… at the young age of six hundred.