you know how hes half human/darkin? is his darkin part armor or hardened flesh? does rhaast when he takes over harden kayns skin to make it more weaponized? plus if kayn drops the scythe will rhaasts effects go away (the armored part on his hand holding the head of the weapon)? how exactly does taking over a host work?
Kayn canon didn’t ever wear much if any armor because he found that it both got in the way and that he was good enough at fighting to not need it. So the darkin parts are hardened/molded flesh due to Rhaast’s influence/corruption.
Kayn doesn’t necessarily have to perpetually hold onto the scythe (he can set it down if he needed to use both hands for something) but he can’t just get rid of it. He is now connected to the scythe and there is a battle of wills and power between he and rhaast for control over his body and soul. If you were to fully cut the connection between him and rhaast at this point, Kayn would be messed up. Those darkin corrupted parts would not reverse to his old parts of his body, and he’d most likely lose his arm and eye at the very least.
We don’t know that many specifics of how host takeovers by darkin work, but we do know that Kayn is the first host that Rhaast has ever tried to take over (most others he has simply found unworthy and killed). Each darkin we’ve seen so far kind of does their own thing with their hosts, but it generally involves them dominating the host’s will, their soul/spirit, and then using some form of blood magic to craft the flesh into a more pleasing/suitable form for them.
I personally think it it's hardened flesh, like Rhaast corrupted that side of Kayn so it can't be undone unless Rhaast is banished.
Seems like that part of Kayn's body is hardened flesh or maybe something similar to toughtened keratine, like nails. If the design that we were shown not so long ago was correct, Rhaast was the only one wearing armor, and his seems to be made with blood magic by twisting the body, made by Rhaast mostly to resemble his former body, as he doesn't seem to be too worried about Kayn's protection.
If Kayn were to drop Rhaast, the effect shouldn't go away as its a body transformed by magic, but it does seem that upon vanishing Rhaast and ascending, Kayn becomes capable of undoing said Blood Magic.
Judging by Aatrox's bio, taking over a host means overpowering its control over the body and relegating said conscience to the darkness Darkin live in during their imprisionment, unable to move, breathe, see, hear or anything, just darkness, solitude and oblivion. Once Kayn gets stabbed and his conciousness lost, Rhaast takes over and modifies the rest of the body with his magic, becoming the sole owner
