Welcome summoners! This is my Kayn guide, and I appreciate all feedback. Kayn is an extremely strong mid-game jungle champion, who tends to carry until your ADC can take over in the mid- to late-game. However, Kayn has an extremely weak early game, until he chooses his form. Without further ado, let's jump right in to learning how to play a strong jungle Kayn.

Pros + Strong champion + High damage + Strong overall - just gets stronger with form + High movement speed + Self peel + Playstyle options + Can become extremely sticky + Really good at farming + Really good at counterjungling

Cons - Easily shut down by crowd control - No real gap closer - Lacks hard crowd control - Is hit hard by Grievous wounds - Has a hard time when behind - Has a hard time in a 1v2 situation if below 75% health

Flash Is a must have for nearly all champions, no matter what role. You should have Flash in every game as Kayn.

Smite is an absolute necessity for all junglers. It helps with your early clear's speed, and is a must for stealing objectives like baron nashor and the dragons.

Your Reaping Slash will be your main clear tool for both shadow assassin Kayn and Rhaast. First you dash, dealing damage in a line, and then you do an AOE slash. Both dash and slash have a 65% AD scaling, meaning that if you can land both the slash and the dash, you do 140 (+65% AD) twice (at max rank). Make sure you land both. Your Blade's Reach as a Shadow Assassin is your main engage tool. It's your only true skillshot, and Kayn's only cc. Shadow Assassin leaves behind a shadow that casts the ability for him, allowing him to move around during its cast time. Shadow Assassin will slow enemies hit by the ability. As Rhaast, your Blade's Reach is a powerful cc tool: you knockup enemies in a line. Your main engage/disengage is your Shadow Step, which allows you to walk through walls. This means that Kayn has interesting ganking potential, and can make most wards useless. This ability is also very good for your jungle clear, as it also heals you for 160 (+40% AD). This gives you some much needed sustain. As Shadow Assassin Kayn, this is empowered, lasting for much longer, removing slows, and granting Shadow Assassin +70% movement speed if he enters terrain. Kayn's ultimate ability is Umbral Trespass. This lets you infest a champion for 2.5 seconds, becoming untargetable, but not invulnerable. This makes you one of the most sticky champions in the game, allowing you to follow them through flashes and blast cones in much of the same way Zed's Death Mark or Warwick's Jaws of the Beast can. The shadow assassin's ability's range is increased, meaning you can ult into an enemy from farther away. As Rhaast, you heal for a % of the target's maximum health, so make sure to infest the tank if you're low. Both instances of Umbral Trespass deal massive damage upon exit, so if you have nearly all your health and have used your R as an engage tool, make sure to infest the target you want to kill.

In terms of Runes, Shadow Assassin Kayn has some options. However, an extremely strong Keystone Rune from the Domination Rune tree to run on Shadow Assassin Kayn is the Dark Harvest Keystone. As a jungler, you benefit extremely well from having an empowered ability, and as you farm throughout the game, the bonus damage from Dark Harvest only increases. Alternitively, you can take Electrocute instead. After this, you want to chose Sudden Impact, Eyeball Collection, and Relentless Hunter for the movement speed and free lethality. Did I mention free lethality?

For your secondary rune page, you want to choose either the Precision tree, or the Sorcery tree. From the Precision tree, you can choose Triumph, because you will be at the center of many fights, and should have a high kill participation, and would benefit greatly from the takedown healing and extra gold. You would then pick Coup de Grace for the extra damage onto enemies you've already chunked. From the Sorcery tree, you can choose Celerity for the extra movement speed so it looks like you're literally everywhere at once, and Waterwalking for the late-game scaling Attack Damage.

For Rhaast, Kayn's counterpart, you want to take a slightly different rune path. For your keystone rune, you want to take Electrocute. This is because Rhaast is more of a duelist, and you will very easily proc this keystone with your Blade's Reach- Reaping Slash-aa combination. The rest of the primary rune path is the same as the Shadow Assassin build: Sudden Impact, Eyeball Collection, and Ravenous Hunter. You can alternatively take Relentless Hunter instead, to increase his map prescence.

For Rhaast's secondary rune path, you want to take the Resolve rune tree, and run Bone Plating and Overgrowth or Conditioning to give him more survivability and some peel for his ADC. Revitalize is also a good choice. You can also take other paths from different rune trees as well: Triumph, Coup de Grace, Celerity, and Waterwalking are all viable choices.

Update 8.13: changed Gathering Storm to Waterwalking, because why are you playing for late when games end in 20 minutes anyways?

As for items, Kayn as the Shadow Assassin will want to build pure damage. Something like a Yasuo, but instead of crit chance, you want to build damage. You want to hurt. Of course, for your first item you will rush your Skirmisher's Sabre, with Enchantment: Warrior. Stalker's Blade is still viable, just not as good.
Next, you build a Duskblade of Draktharr for the damage, lethality, and the ability to be a walking control ward with its passive. The first thing to do after buying your duskblade is buying a Youmuu's Ghostblade to give you more lethality and movement speed out of combat.
At some point between your jungle item and your Youmuu's Ghostblade, you want to finish your Mobility Boots. You do this because the out-of-combat movement speed means you can gank so much quicker. This movement speed buff also stacks with your movement speed increase you get from your Shadow Step, meaning you are lightning quick. It would not be a stretch to hit speeds of 700 while in a wall. The enemy will think that you're everywhere at once! You could gank midlane in a couple of seconds™ after finishing off the bottom lane.
For your final items, you have some options. you could build an Edge of Night, which gives lethality and a spell shield for 7 seconds! And either a Lord Dominik's Regards or a Mortal Reminder for the armor penetration or grevious wounds. Finally, against a team mostly comprised of high AD carries, you may benefit from building a Thornmail or a Randuin's Omen, which gives you some armor, health, and the thorns passive, returning some of the damage to your foes. However, you want to avoid needing to build these items, because it delays your damage.
Between all these items, there's one item that most assassins shouldn't need, but really benefit from having. That's your Guardian Angel. This gives you a revive every 5 minutes. If you find yourself needing to buy this, think about why you are, and if you're making stupid moves, you should stop making stupid moves.

As Rhaast, you will be innately tankier, and will therefore want to build on that fact. Just like the Shadow Assassin, you build your Enchantment: Warrior. Then, you build a Black Cleaver and Ninja Tabi. Following that, there's some options.

The bruiser option: a Spirit Visage, and a deadman's plate. If you need some items with sustain: Death's Dance. Keep in mind, these are not set in stone. You can mix and match these items. If, for example, the enemy team is comprised of physical damage champions, you may want to exchange your Spirit Visage for say a Thornmail or a Frozen Heart. Another option would be building an Adaptive Helm instead of a deadman's plate if their team relied more on magic damage. Your final items are really up to you. Some options include a Frozen Mallet, and Iceborn Gauntlet, a Guardian Angel, a Bloodthirster, the Edge of Night, a Mercurial Scimitar and even a Sterak's Gage. A triforce is also an option. It's like a mix-and-match dress-up day!

PATHING

Kayn has an great clear. He isn't reliant on a specific buff to complete his clear, but the jungle camps can cost a lot of health. Kayn doesn't necessarily need a leash from any lane. Kayn can clear the raptor camp with ease, then going to the red brambleback to finish a full clear for the red side. From there, Kayn has some options. If he's on the blue team, he can gank mid coming in through the walls in the river, or coming down bot side for a nice little flank from your laners. If he is on the red team, he can do much of the same with the mid lane, or gank the top lane the same way he would for the bottom lane.
When you're ganking, you want to make sure you hit your W. This slows them, and gives an oppurtunity for your laner to follow up with damage and cc of their own. You can actually cast your W from inside a wall, giving you a little more surprise and more chance to hit them.
Once you have your form, your ganks change slightly. As the Shadow Assassin, you will still want to land your W, but as soon as you tap the key to launch the skill, you can immediately press Q to close the gap between you and them. This combination can also apply to any jungle camps, and will pick them off very quickly, improving your clear speed drastically.
As Rhaast, you have something of the same combo, but you want to make sure you come out of the wall a little closer to them, as the range on your W as Rhaast is shorter than that of the shadow assassin's.
If you've completed a full clear of your own jungle, and gank oppurtunities do not appear, you should start counter-jungling or pressing objectives. Camps that junglers often leave include the wolves and the krugs. Raptors are often open after a first clear, as most junglers don't have good AoE clear. You may even be able to steal that camp a couple of times. The raptors are a massive gold and exerience well, so you want to keep good track of yours and theirs. These are easy pickings, and give perfect oppurtunities to throw down a deep ward.
Kayn has a hard time early, but it's not that bad anymore with patch 8.6's buffs to his damage abilities. He doesn't have a nice passive ability that gives him a huge benefit early like Hecarim or Kha'Zix, so most of your damage will need to come from your abilities. He is relying only on your mechanical and game skills. Only when he chooses his form does he get this nice passive. This means that you have to be very careful and only engage on fights you know you can win -- or you know you can get out alive. In other words, get your laners to engage and go in and hit as many enemies as possible in the couple of seconds the fight is taking. This then means that you want to farm quickly and effectively to be able to get that lead that you will need to survive these fights you need to engage on. If you already have a kill or an assist, it's very easy to snowball from there, so you can counterjungle or press objectives very easily.

As Shadow Assassin Kayn you're a pure damage assassin. It's in the name. You assassinate. Point finale. You kill them before they can do anything. When you hit them with your max rank Blade's Reach, they will try and run. But they are slowed, so they can't. Then they die, because you use your Reaping Slash to get close, and then hit them with a Dark Harvest empowered auto-attack to deal HUGE amounts of damage. With your completed Infinity Edge, if you decide to do so, you could crit and deal EVEN MORE damage. You want to try and do this whole wombo-combo within 3 seconds, while your passive (which deals bonus magic damage within the first couple of seconds of a fight) is still up. Honestly, its something of a jump-scare because of your movement speed. They get a little bit of a warning, because a little dark circle shows which wall you're coming out of, but they won't see it. Let's be honest.
So, back on topic: You're assassinating people. It's very easy to counterjungle once you have your form, because you're going to end up killing them before their team is there (unless you're behind or it's a super tank jungle, then you should be going Rhaast). When you assassinate, go in through the walls and scare them. They won't have time to react, and you kill them and you get out ASAP, because their team is coming. Even if they're not, they're coming. They know where you are. Go farm some more for you Dark Harvest passive. Rinse and repeat. When the time to go and fight as a team comes, you're definitely not front line. What you want to do, is try and flank the enemy. This takes some coordination with your team, because if you assassinate the carry and your team can't follow up, you lose the fight. You're still putting out that crazy damage, and after ulting one of them because you got to them with your Blade's Reach- Umbral Trespass combo, you can just slash through them all with your Q, and they can't do anything about it.

So, let's recap:
Assassin Kayn assassinates. That means making picks on enemies and not dying. During the laning phase, if you spot their jungler, you should be able to kill them. If you aren't certain you can, notify your laners so they can collapse on the enemy if anything goes wrong. An easy pick, usually, is seeing a roam from mid down to top or bot and a roam from the support to mid. It's somewhat harder to make a pick onto a top laner because they're usually tankier. In the midgame, if you see a fight breaking out, drop your farm and get there. You need to be at most fights, mainly because you are the god of the midgame. Don't worry, your farm will still be there. During teamfights, that means taking out the AD carry before they can plow through your team, letting your own AD carry plow through their team. When you're not teamfighting, you want to be either pushing side lanes, counterjungling, or taking objectives like a dragon. This makes the best use of your time not spent getting kills.
With the newest patches, there won't always be a squishy ADC anymore. That's why you want to try and take out their support if they have a healer, because if the healer can heal the team, you'll probably lose.

As Rhaast, you might play the huge role of disrupting them. This is where you may want to build a little tankier. It's funny, because once you have a Guardian Angel (you probably should if you're engaging like this), the whole enemy team is using their abilities (and therefore wasting their cooldowns) onto you, and you're just going to revive or reset with your ult. You may also possibly get a kill or two out of this engage. Your team should be right behind you, ready to kill them all once you've engaged. If they're not, you should ask yourself, "am I too far ahead?" This is your main teamfight role.
You may be split pushing, and here's where you want to build some sustain. You have pretty good waveclear as Rhaast. When you position yourself beside the minion wave, you can usually get most of them if they're bunched up with your W. You can also usually get most of them if they're coming at you straight from base. The few you miss can be quickly picked off with your Q, and then you want to hit that tower with every auto-attack. This is a very viable role as Rhaast, because you have a very easy escape: Walls don't exist. You run away directly, no need to worry about any pesky walls (for 5 seconds). Then, you rejoin your team and take that teamfight that they've probably abandonned to team up on you. Then you take a different turret with your team.
As tanky Rhaast, you're something of a frontline, but you still are a little squishy. Now that you've build your armor, and you have a reset with the Guardian Angel, you can soak up some of that damage! Also, your healthsteal passive ( the darkin scythe:rhaast) heals you for about 45% of the damage you deal. You get really strong like that.
But you will still die very quickly if you don't have your team with you. If you're trying to win a 1v5, you should wait for your team and force the teamfight you should win.

At this point, you should have the advantage. This means that you're up in level, gold, and farm. To win the game, you should focus your thoughts back onto objectives. This means extending your advantages.

Case 1: You are a Shadow Assassin, your bottom lane did really well, your carry has 3 kills, a significant farm advantage, and a completed item. And for some reason, they don't have tower yet. You want to stick around there and help them push that tower. After that, you want to get them to go and push another lane. This usually means that your bot lane roams mid. Now, there's the possibility of a teamfight happening. You should have vision in the river, so you should be able to see when their bot lane is coming up for that teamfight. Well, what can you do now? Let's take stock of SA Kayn's kit.
1) Stupid high damage
2) Slows with gap closer
3) Ignores walls
4) Stickiest champion in game
5) Insane movement speeds
6) Extra damage passive is refreshed upon ult
With this in mind, you may be able to assassinate their carry. This gives you another advantage. You may be about to enter a 5v4. This is a very advantageous situation. You should be able to take at least 1 objective. In this specific case, the objective is probably midlane turret. Many people in this situation will chase needless kills. Before you do this, however, you want to think to yourself, "do we need to dive them? Will they be able to kill us if we take turret aggro?" I've seen many teams try and go for a turret dive, get aced, and lose 2 turrets as a result of them all dying. You don't need to do that. Keep this in mind, if you have a noticeable advantage, you should be able to just take the turret and they won't be able to do anything to your team.

Case 2: You are Rhaast, and the enemy team is going for your midlane turret and your team is very close to you, and you are going to collapse on their team. Your team has an advantage, and you're confident you can win this teamfight. Well, what can you do now? Let's take stock of Rhaast's kit.
1) Stupid high damage
2) Stupid knockup that has a ridiculous hitbox
3) Ignores walls
4) Heals for a percentage of the enemy's health when ulting them
5) NEW! Can queue abilities during Q to make your followup knockup or ult easier, increasing your surviveability
6) Stickiest champion in game
Depending on your build, several different situations can happen. If you've gone a tanky build, you can engage hard onto the enemy and survive until your carries get to the fight. This will hopefully waste most of their cooldowns on you who will survive and disrupt them. If you've gone lethality-lifesteal, you can still disrupt them, waste their cooldowns, but you'll want to get behind your tanks ASAP. If you play it well, you should win the fight. If you went tank (I.E. if you're not trolling) you join that frontline.

Case 3: You don't have a form yet, and a skirmish is breaking out in the topside river. You're level 4, and the enemy jungler is level 4 as well. Your top laner is getting collapsed upon by their jungler, and your midlane is on the way. First off, ask yourself, "should I go there? Will I die and set myself behind?" Frankly, you should always be trying to get your form as your first priority. So if a fight is breaking out, yes, you should be there, but no, not if you will die. A good way to survive these risky fights is to walk up to the fight, and poke them a little bit with your W. Stay far from the fight until you're sure you can survive. Then, when you're sure you'll survive, Q through the fight to get as many orbs as possible. In the early game, Kayn thrives on these sorts of fights. This is how he gets his form before 10 minutes. Take part in as many fights as you possibly can.

There's a tricky mechanic that you want to abuse that will increase your survive-ability: you can flash or Q into a wall when you cast your E, instantly giving you the heal. Abuse this when you can.

I am by no means the best Kayn player in the world, nor am I the best jungler ever. I won't tell you when to go for objectives, nor should I. I am no coach. What I do know is that if you survive the early game, and play smart, you should do reasonably well as Kayn in the jungle. I appreciate all feedback and discussion, so please leave an upvote and comment below. Good luck in the Rift! Be sure to check out my jungling guide, here.