After multiple games on him I genuinely believe this champion is trash. I've played more than enough Rhaast games to see that he doesn't do shit for damage and myself and friends have also found that Assassin form is a terrible assassin.
He's supposed to be good for what you pick but Rhaast is an awful bruiser, none of his OH WOWSERS %MAX HP DAMAGE things do shit because its all mitigated by armor and he has no method other than items to shred armor on top of mediocre damages all around.
Assassin Kayn is just straight up outclassed by other assassins.
I don't want a champ that can be two things depending on what you want BUT be awful at what you choose. I want a champ that works and I wouldn't care if it's shoehorned into one role from the get go.