Nithael is one of angel stationed in Earth after The Darkness rising.
At some point, he tracked if Lucifer presence detected in a park, only to find if he is inside the Castiel body. When he tried to killed him with an angel blade, Lucifer tell him to stop it because he claimed his coming is for saving the world from the Darkness as he is only the hope to beat her. However, Nithael doesn't believe him and trying to kill Lucifer and saying if the Darkness is invincible. Nithael action make Lucifer to molecularly combusted him, killing him.