Kayn is actually the apprentice of the Lord of Shadows, so he knows shadow magic.

I don't know where he got a scythe from, but it's actually Rust, the darks have no body, weapons are the body, they will erode the soul of the host, Aatrox should have been transformed into a sword Demon, but Kayn still maintained a symbiotic state, and both sides were fighting for the dominance of the body. Therefore, when he is not transformed, Kayn talks a lot to himself, similar to Kindred. Given that Kayn was cultivating in the shadow stream, he certainly wasn't a good person, and liked to kill and destroy. When killing Zed in normal form, Rust will say to Kayn: "Zed is dead, baby, the last lesson of Zed is to not be like him.