Good question i am a kayn one trick(fan boy). Who has been playing the champion since season 10 so I know a thing or two about kayn at the moment of making this i have 290k mastery points and am mastery 7 on kayn and got inspired by the new "fun" patch for assassins to try this build out that Karasmai did a video on at the start of season 12 here is the link i recommend i learned almost everything i know about kayn from himKarasmai video

So it all started when I tried playing kennen top. I was dealing good trade damage and successful mid game but... My creep score was horrible.So I tried mid lane.Same problem so one of my coaches gave me the idea of playing junglers in mid lane. Then I remembered Karasmai did this a couple times! So if the kayn god Karasmai was able to do it and make it work why would't I be able to do it? After all I used to play it myself did this patch really make it that unplayable? Well in order to find out if I really know all that much about kayn I made a guide before I even tried it this patch and it took me two hours! I will be updating this page weekly with my discories and if it goes inactive that means that this build was horrible and so was the idea of kayn mid after the major assassin nerf.

Blue form:Axiom Arc: go when in enemy team is very squishy and you can one shot them otherwise not recommended
Youmuus Ghostblade:Go this item when you need more map prescence
Serpents fang:only go when they have shields or earth dragon soul
Serylda's Grudge:kind of a must have this patch because they will build armor items but situational because its a good 4th item whiich the game might not last that long
Umbral Glaive:go umbral glaive when you dont need any of the other lethality items
The Collector:go when they have bruisers with high hp like chogath
Edge of Night:Go this item when going into a champ that hasw a cc engage like ahri

Black Cleaver:Good item to get after manamune to amplify your damge when they are building armor and gives you a speed boost for positioning in teamfights
Thornmail:for building armor and grievous wounds if they are mostly ap champs dont bother going this item
Chempunk Chainsword:go this item against champs that are ap and heal
Spirit visage good item against ap champs the best item for rhaast that has mr
Steraks Gage:go this item for tankiness with a bonus damage boost would not recommend if they have serpents fang
Deaths Dance:kind of a must have item you dont have to rush it but you really should build it at some point its like triumph but a good ad/armor item also great against burst units need i say more?
Maw of Malmortius:just go against a team with oneshotting ap champs it will save you a lot of health
Frozen heart:go against evasive auto attack champs if they are ranged and you manage to get close to them unless they are like vayne where they can dash often then its a pretty free kill but all in all good paired with muramana

Its pretty simple you just gotta poke with your w and paired with arcane comet you should be able to make some laners submit to you. But you gotta start q for the wave clear. You can get some good poke against weak level 1 champions but i would not recommend you do an extended trade since that would attract the enemy junglers attention early. you should play cautious of the enemy jungler until you get your e at level 3, and go rhaast into melee mid laners and an enemy team that you feel like you wont be able to end early against. Go shadow assassin against squishy or ranged champs/mid laners. You are a very strong snowballing champion if you stay mid lane the whole game you are playing kayn wrong. He is a jungler at heart and needs to roam and impact the whole map.

Arcane comet was what Karasmai used for kayn mid at the beginning of season 12 and for a good reason.Kayns W(Blade's Reach) in his base form "Active:Kayn swings hise scythe un the target direction, dealing physical damage and slowing enemies hit by 90% decaying over 1.5 seconds"this is the website i found this description off of arcane comet is a good poking rune which it takes some time to land. Which is why it has such a low cooldown so you get it to land sometimes. Kayns W(Blade's Reach) SLOWS enemy champions by 90% so, it is almost a guarunteed hit so you will get some really good early poking damage out of this rune. I will be experimenting with First Strike to see if there is much of a difference with the damage output and if there isnt i woudld recommend First strike instead as it gives you extra gold and better mid game burst. But for now i would recommend you go arcane comet. Keep in mind though you will have to poke more for form with arcane comet than you would with extended trades with first strike since first strike gives double the orbs for its duration.

Huge thanks to Karasmai for the inspiration.
channel link to karasmai