Just food for thought, since the Darkin are just sealed away evil Ascended as we now know, does Kayn taking over Rhaasts powers make him ascended/Darkin just with his own mind in tact? Put this in story cause it has lore implications.
That's a good question...I don't think he would be a full ascended, since he didn't get the actual blessing from the sun disc. He might just end up as a human with near-Ascended power levels.
When Kayn wins over Rhaast, he technically KILLS him iirc, as stated by a Rioter in a dev blog on Kayn.
But even if he kills him he obtains his powers, like Rhaast is technically killing Kayn and just getting his body. So since the scythe assimilates with Kayn, and makes part of his body Darkin, I'm wondering if Kayn absorbs the Darkin but keeps his mind in tact like an opposite of Rhaast taking over Kayns mind. Which would make his body the body of an ascended with the mind of a human.
I think he just absorbs Rhaasts magical powers.
Its heavily implied that shadow magic absorbs souls to make the user stronger as there was a shadow magic user in 
& 
s color story who fed souls to his blade(and Zed coincidentally got the ability to steal the AD-power of his victims a few months before the release of the color story).
So I think Kayn simply kills rhaast and absorbs his soul like he'd do with any other victim of his shadow magic,but he gains a much bigger power boost from the soul of a darkin than the souls of humans.
