@Turtletrekker covered Harrow and Sun King pretty well with his links. Sun King is a new character from 2018 who was never given a real name (and thus would be easy to merge with the one off Harrow) but who I think fits Marvel's urge to often give us a first villain who is the hero, only evil. Sun King is powered by a belief in Ra, and was a pyrokinetic (that is, he could manifest and control fire). He's basically Day Knight.
As for Mr Knight, it's sort of complicated. But just as Marc Spector has three personalities (Marc, Jake and Steven), Moon Knight also has three aspects. 1) Khonsu, where he is the living avatar of the moon god who powers him (typically portrayed with a large bird skull for a head, like the hallway monster in the trailer here who is probably something similar). This is usually Marc wearing the skull and ancient style clothing. 2) Moon Knight, the protector of nighttime travellers. This is the super suit. Standard issue Moon Knight. Think of it as his work uniform, which is how Marc tends to describe it. Also "his vestments."
And finally, 3) Mr Knight. This version wears a white or silver business suit and a face mask and is basically the public face of Moon Knight when he's dealing with police or civilians. It's both his most human aspect and the one that he tends to wear the most often these days. I've often looked at them as a kind of spectrum, with Mr Knight representing Marc as a human, Khonshu as the supernatural avatar, and Moon Knight as the bridge between the other two.
