Hey there, I'm TheRealOnion from EUW, this is my first guide I uploaded. First im telling a bit about me. Im 21 years old and live in Germany, that means also that english is not my nativ language, so i hope you can be tolarant with me and my grammar. I play League now for 4 Years, since this year started i didnt played ranks, so im still Silver.
Now I will start with the Build:
A few days since the release of Kayn has passed now and i wanted to check how other player thinks about Kayn,so ichecked Mobafire and was really surprised when i didnt saw any Kayn mid Guide. But im playing him nearly only on mid, why I do so will come up later.
So i decided to make a guide for Kayn mid so that everybody can see how this Assassin can win all lanes at once.

This build is still in progress, so dont be mad with me when it is a bit boring. At the moment i just want to fill this build and guide with informations.

Lets Start on the Runes, I tested some Runes on Kayn and in my opinion this Runes had the best stats for the Shadow Reaper. The lethality Build with this Runes means that you deal almost true dmg to all these squishies carrys. That means a kill in less than 3 seconds, befor the passiv rans out. More of this below.
I also testet Scaling AD, flat AD and precision Runes. I also testet some combination of these. The most Dmg you will gain with AD Quinessence and precision Marks, but when your passiv is on Cooldown your damage will drop, also will these runes dont help you farming.

Another important Point for Kayn were the materies. here we have again some possibilities we can go for. Lets start in the Cunning tree

The probably most important chapter will be the items, everyone have different opinions here, i have choosen all these items with a reason, most the time the reason seems to be clear.
Lets start with the items you choose on start:
Here i prefer to take Doran's Shield and fill with potions. since Kayn have a weak early but still need to get close to the enemys you need the additional health regen from this.

As soon as possible (at first back when you can pick an early kill) you go for Phage and Boots. To avoid to buy 3 pots everytime you need to head back you take the Refillable Potion. Why to take boots should be clear, but why Phage and not Caulfield's Warhammer still need to be explained. Both give you some AD, warhammer more than phage, but the thing you want most in the early game is sustain, so you will be pretty happy about the 200 hp [phage]] you gives. Also its passiv allows you to get close to the enemy to get some more points for your passiv.

After you finished the Items above and you reachend from this point around 1500 Gold you should be able to choose your form. head back now get Kindlegem and Mobility Boots, make the transformation into the shadow assassin and head back into lane, till you get enough gold for Black Cleaver.

From here i will just write one sentence per item

Youmuu's Ghostblade: Movement speed and lethality, works fine with your Shadow Step

Death's Dance: the passiv give you more sustain and the healing is pretty good while fighting enemys with high stustain

Duskblade of Draktharr: after reaching this item you will be able to one shot the enemy carry

Edge of Night: use the activ while you were in the wall, get out of the wall take the kill safe, allows you to take down enemies which would use their cc on on you and one shot you (like Lux with her Q and her ult)

Guardian Angel: if you getting one shootet take this for another chance

Maw of Malmortius: when the enemies have a heavy AP team, or the AP carry is fed

Ravenous Hydra: for some more waveclear

Lord Dominik's Regards: when the enemies have a team with atleast 3 tanks its a must have item for you, if they have 2 you can take it but that you have to decide dependeing on the situation.

Start with Blade's Reach allows you to get your passiv stacking for the first minute, if you would start with Reaping Slash you would take a lot of damage withou the chance of winning the trade. So start with Blade's Reach to stay safe while collecting the energie you need for the transformation.

Max order is R>Q>W>E
Why maxing the R first should be clear
You maxing your Q next cause it has the most damage from your kit, also allows it you to get close to enemies
Your W ist most used for the slow and in the beginnig to gain the needed points for your passiv
You will maxing your E last cause the heal isnt that enourmous, even in late, you only use it to roam, so you can travel between mid and bot in less than 6 seconds, then you pick up a kill on bot, and when its cooldown is over you will be back in mid.

Flash is just the classic spell for nearly every champ, Kayn is there no exception, only thing here to tell is you can activate your Shadow Step and Flash straight into the wall. That will heal you instant while you still can run away

Ignite: i prefer to take this for a higher dmg, also its grievios wounds allows you to take down the ADC when he reached a high lifesteal and got nearly immortal aslong you cant cc him

Ghost and Teleport: i wont take one of these cause you were already fast enough, you can travel all over the map in just a few seconds, so these would be end in a really big waste

Pros:
One Shot the Carrys
Very high mobility
Fast engage/disengage
Good Wave clear in mid/late game

Cons:
Very short range at the start
Less damage before transformation
Easy to counterplay in early