It's interesting seeing a Ring-wraith just on the point of turning wraithy! It seems like Karbûkhâd is gradually realizing he's in too deep, even if he doesn't yet know the worst of it. It seems so very ominous, especially this: "he could not turn back without Sauron’s exact orders. But Sauron was in Númenor..." If he can't find the rest of the Nine for some reason, he could be wandering around for literally centuries until Sauron bothers to change his orders . . .