Try this riddle out.

[Despotus]

Ok the definition of a castigator is one who castigates or corrects. So castigate=correct. But that doesnt make sense with the rest of the phrase. I think that in the context of "People, whom the black air so castigates?" castigates means corrects. So People whom the black air so corrects ?

Your clue is not helping me much and this is all i can think of at the moment.

Castigates is closer to Punishes than Corrects. But, I'll take another look at this riddle later, gotta go to work.