It's said in the fluff that Haarkon might be a Night Lord because hes such a good raptor commander. If you want him in 40k
, I think he should be a Legion Consul Champion with paragon blade and jump pack. Model wise, I think the kharn model would look cool. Use a night lord looking head, put a jump pack on kharn, give them a chain glaive (easy to make, but model a chaos csm
old school squad chain sword on the top and extend the bottom. I can see be gifted a chaos spear during the long war.
Kinda looks like these combined:
The Kharn model is a great base for a jump pack champion with a chain glaive, just paint them up Sons of Horus colours and it would be exactly like Haarkon. In the ADB Night Lord short stories, it should a raven guard change side. In Unremembered Empire, the night lord all hate each other. I can see a night lord captain defecting to the Sons of Horus. Hes the kind of no nonsense guy who would say: "Yeah nah, I'm having none of this undisciplined baby killing - its so unprofessional. I want to be a Sons of Horus, those guys get things done."
Anyway, that's how I would do it. Hope you show us what you come up with