I am diamond level jungle main (peaked masters 200lp) and i like this champion but i have seen that almost every kayn i have played with or against doesn't have a good idea how it works so i figured that i should make a guide that could help people understand him better and how to play him on different stages of the game and what to build. Also I added the new runes and covered them so you can use this guide on season 8.

So for the starters you know that he is a pretty random champ so im not a master at him but i have him figured out pretty well so here we go. Lane phase; kayn has really good and fast clear and you should use this to your advantage. I like to go with the clear path of taking raptors first, then red, then wolfs, blue and finally gromp. You will hit lvl 4 from this clear and you should look for a gank at top/bot depending which side you started. If the enemy is pushing just E through the wall next to your tier 2 tower and slow them with W and this is where ghost is really useful cause even if they flash you can keep up with them. On top lane this is really easy kill if your top laner has any form of cc and should be pretty clean first blood. If you are on red side and ganking bot lane then you should go through the same wall next to your turret and use the same strategy but you should focus only other and not get greedy for double kill. if your lane is pushing then you should go through the wall next to tribush and try to flank from there, this is much harder but you can get the kill if you have stun on your laners kit.
After doing this you should probably go for B if you don't see good gank on mid lane and you should buy skirmishers and boots and some long swords if you have money for them. After this you should just power farm and try to counter gank because it's super easy with kayn's E and try to gank if your lanes are getting pushed. I'll try to get some footage of my gameplay to here at some point. Anyway at 20 mins you should have at least 100+ farm and have death's dance and the darkin form. Sometimes you can just end at 20 mins if you have fed your laners enough but if not then im gonna tell you about mid game.
Mid game is pretty simple for kayn, you are excellent duelist and you can kill anyone and leave the fight with full hp because your ult heals you to full with the items you should have by then. Try to look for picks and keep farming your jungle and help your team with fights with your knock up W and insane dmg and tanking potential. You should have won by 30 mins because your mid game is super strong and enemies wont expect anything from you. If however your game goes over 30 mins and in to the late game, you better be prepared to make shot calls and pressure baron or split push if it's the only good choice. I would recommend going with your team and starting fights with your W and tanking enemy ults because you can just heal to full by ulting some tank or even squishy and killing them in the progress. So in conclusion kayn is play maker and fight started with his good aoe knockup and insane pressure of his E, I hope this helps you to play kayn and clarify what this champion is all about. Frontline damage soaking heal machine, better version of aatrox. 9/10 would main..

So because kayn is frontline bruiser who can heal more than vladimir with 4 soraka's in his ******* and literally goes inside enemies and leach 30% of their hp and gain full hp for himself, his items are pretty obvious.
First of all jungle item, Skirmisher's Sabre- warrior, is must have because the dmg spike is insane and the red smite is super op on this champion.
After getting this you should rush ninja tabis for tankiness and movement speed which helps you stay in fight for longer and chase enemies. If your enemies have like 4 ap or a lot of crowd control since cc is the best counter for you as it stops your healing and damage output for a while.
i would say the most core item is black cleaver because it gives so much mobility and damage with the armor pene.
after black cleaver you should get guardian anger for dmg and armor which is super useful when you are in the middle of all the enemies after ulting or using E. This just gives you more options because of the revive and dmg to fight tanks and burst down squishies.
the last 2 items should be hp items like steraks gaze because it gives you a lot of hp and damage for low cost and gives you huge shield which helps you sustain the dmg and keep healing and now it gives tenacity when the passive procs which helps you to not get locked down. then you should get either thornmail or spirit visage depending if they have ap or ad heavy team, thornmail is strong because it gives a lot of armor to protect your hp and it also deals damage and applies grievous wounds which helps you to win long fights. and the spirit visage is obvious item if the enemies have ap damage because of the passive, "all healing on you is increased by 30%" which is insane for a champion like kayn and you should consider buying it if you have like soraka in your team even if the enemy team has only like one ap dealer.

Take dark harvest so you can get more dmg the more you farm and you can burst enemies with this. Dark harvest is good on kayn because you will be fighting long so every kill gives you the edge to kill next one. Then you want to take "sudden impact" because it gives you burst of lethality everytime you use your Q, which is on very low cooldown. Then just take eye ball collector for dmg. now we get to the part that made me really excited, "ravenous hunter" is like made for rhaast because it gives him even more healing and is very helpful in the mid and late game. Next you want to take triumph for it gives you sustain and gold, in a mid game fight if you get a kill with your ult, you will heal to almost full hp and in late-game the 15% heal is huge. the next one doesn't really matter but the tenacity one is pretty strong because your biggest counter is cc lockdown and this tenacity helps you to spam Q and not get lockdowned. i think these new runes are better for kayn and gives him more power in late game. you also get +11 attack damage from the runes which helps you clear the jungle faster.