I think Kayn might be a little stupid
See, what makes a weapon powerful? In his case, a scythe, has only up to three factors from which its power can derive. 1: It's sharp. This is a necessity. If it ain't sharp, it's a weirdly shaped club. This can also be achieved through various ways of smithing the blade. 2: It's durable. This is also a necessity. If it breaks in combat, it's broken. And I don't mean the bad good kind of broken weapon, like Riven has. No, I mean broken as in 'not usable'. This can also be achieved through its construction. Like, take oak, instead of balsawood. Or metal. 3: It has some magic factor. Like, idk. Maybe the blade is so hot it burns someone when it cuts. This can hardly be achieved by the means of production and is probably the reason Kayn wanted a Darkin weapon in the first place. I mean, I don't know if Rhaast even does this. We never see him do anything the like, so, idk.
BUT So far, the only thing that makes a Darkin weapon better than other weapons and gives them their special properties is, you guessed it, the Darkin inside. Now. What. Does. Kayn. Do. If. He. Wins.
He kills Rhaast.
He literally kills the one thing that makes his weapon more powerful.
What an idiot.
Now, I'm not denying that Kayn becomes more powerful himself after winning the struggle. But I'm not contributing that to his weapon. I say it's actually the opposite. Yes. Kayn drives his power from his own bloated ego.