I think you missed a bit of lore hiding in the VO and the short story. I'll do my best to explain this without spoiling anything.
So, Rhaast is not from the Odyssey universe. No corruptant is -- he is alone, having fallen through the Ora Gate at an as-yet (pay attention to the special loading screen tips when the mode goes live!) undetermined point in time, and can't return on his own. Being a corruptant, his ultimate goal is to consume all of existence (this is why he sounds like he wants to eat people) and and then leap into the Dark Star itself, ending time, thought, and matter. However, the Dark Star does not exist in Odyssey. This "Ora" stuff does instead, which is biological fuel that enhances people and technology. In order to reactivate the Ora Gate for his own purposes he needs both a body (he somehow got trapped in this scythe, again pay attention to the loading screen tips!) and a TON of Ora, so he subtly influences Kayn and makes his unbidden ambitions take over his mind. Twisting his youthful anger into an obsessive need to adsorb the Ora of other sentients, Kayn takes Sona, someone who can speak to Ora and who knows how the Gate operates, and attempts to 'open' it and gain 'unlimited power.' This, of course, is a ruse. Rhaast is lying the entire time -- he doesn't care about Ora as anything but a tool.
Sona escapes, and Rhaast and Kayn end up getting stuck with each other for years longer than either of them intended as they hunt her down. She joins the Morning Star, Kayn starts chasing the crew, and bingo bango here we are.
I don't think anybody knows what Rhaast really is. To understand his true self would be like a normal human comprehending an eldritch god -- even the general concept of the alien, sentient Dark Star would break an average person's mind. Jarvan is just a mortal emperor doing Mortal Emperor Stuff, and Kayn starts actively disobeying him the minute he picks up the scythe. Dude has no idea what a Dark Star is... we do because, obviously, we have Player Knowledge. They do not.
As for the Ora Gate? I know what it is and how it works, but I'm not talking ;)
Annnnnd... yeah, I didn't spoil anything! Hope that helps!
Special Interactions = Rhaast in this universe is affiliated with the Dark Star.
