Kayn needs some buffs and tweaks
His kit overall is cool, but a great deal bugs me about the champion. He is meant to be a Carry and as of right now, he's not carrying. Aside from being a mostly melee champion, he has some reach, but other than that he is lackluster. His early game is weak. I get most carrys are lategame, but Kayn never hits a sufficient enough power spike to validate him. Yes both transformations offer a spike, but nothing at all to validate him being played over other champions. He is Yasuo without the dashing and crit. Yasuo without the dashing and crit is nothing.
I have tried various builds against various champions and nothing you do can validate him as he is. His Shadow mode offering more damage, but not enough and is trash tier on release. His Darkin form is better, but not by much. His shadow form is being out damaged and isn't fast enough to make up for it. His Darkin form is easily out tanked/out skirmished and though it has more sustain, your not keeping up with the xins and the yasuo's and the vaynes.
Have you ever tried to face machine gun tank LuLu with Kayn. It's depressing that some flower fairy is outbursting/outspeeding/out damaging/out kitting/ a "reaper". Her utility with attack speed/tank items is crazy. She is fullfiling the fantasy better than Kayn ever could.
Kayn's q and W dont activate on hit affects as neither one is considered an AA, I understand w, but Q should. I keep being knocked out of walls when in e and someone attacks me in this mode, basically making it useless. His R allows the enemy team to walk me into their turret or their team and you get straight jacked up.
Lethality barely works on him. Doing anything with crit means giving up the cooldowns needed. Tank means 0 damage. You can't win. He needs to be buffed. He is a skirmisher/assassin. The fantasy and the actual don't match and is a huge let down for the scythe champion many of us wanted. That is fairly evident with his 41/43% win rate as a carry. I got my first S- on the champion with a score of 3/1/2 and that is fairly sad when most carrys aren't getting s ranks without at least like 17 kills.
That's just my piece as of right now.

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With the option of replacing cinderhulk for warrior if you need to be more tanky (Maybe Cinderhulk might even be a bit better considering his %hp on everything).
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