Kayn is one of the newest champions to enter the Rift. His character's main feature seems to be his mid-game transformation. I've never played as Kayn myself, but I've seen several teammates and opponents use him.
To better understand him, I viewed his abilities in the client. Unfortunately, the descriptions provided there don't give much information in regards to how his passive actually works.
From what I understand, Kayn will transform into either his Darken form, or his Shadow from at some point during the game. Also, I've seen a teammate mentions that he intended to use Darkin Kayn after seeing the enemy's team composition.
Is one form is better suited for dealing damage while another excels as a tank?
I'd like to know what the differences between Kayn's two forms are, and what triggers his metamorphosis.