What is the relationship between Azathoth and Nyarlathotep?

I see it is said Nyarlathotep is a servant and messenger of Azathoth. I just wonder, does Azathoth know that he has a servant? or does he really have messages that he would like to send out by a messenger? Does Nyarlathotep actively obey and follow a master, or he just does anything he wants and call it Azathoth's will?

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