Welcome to my all-round kayn guide! This guide is here to layout the basics on what runes/masteries to take, as well as builds for both laning and jungle kayn. I've been playing kayn quite a lot since his release, and i am currently sitting around 90k mastery points on him, and this guide is to set out what i typically build/recommend on him for every game. He is strongest set out in the jungle, but can be a fun pick for laning, i do not recommend jumping straight into ranked with in lane kayn.
Kayn is strongly known for his ability to counter jungle, he E makes it easy to take specific paths in the jungle that can pass around commonly warded areas unnoticed. This allows him to gank from unexpected areas as well.

Flash/teleport are the standard masteries for laning kayn top lane. For midlane, other options would be cleanse, ghost, or ignite in addition to flash, All depending on both personal preference and the matchup.

In the jungle smite/flash are the only masteries to take. Although, i have seen smite/ignite work quite well for twisted treeline jungling.

ABILITIES:
Q - Reaping slash.
Kayn's q is a 2-part single cast ability that is the core of his jungle clear. It has a set base damage+ad scaling damage that applies both on the dash, and the whirlwind slash he does at the end of it. Upon changing to darkin form, this ability will do % of target's max health as damage for both portions, scaling off of your attack damage.

W - Blade's Reach.
Kayn's W is a point and click skillshot that shoots in front of him in a rectangular shape, dealing even damage (scaling off of attack damage) to all enemies hit by the area marked. Upon changing to shadow assassin form, kayn will no longer be stopped from movement during the casting animation for this ability. Upon changing to darkin form, this ability will also apply a knockup to enemies hit.

E - Shadow step.
Kayn's E is the ability that gives him the ability to walk through walls. Upon coming in contact with the first wall after activating this, kayn will heal himself for a flat amount + more scaling with his attack damage. Upon changing to shadow form, this ability will also remove slows and give an even stronger movement speed boost upon coming in contact with a wall.

R - Umbral Trespass.
Kayn's ultimate can be activated on enemies he has recently damaged, this ability temporarily makes kayn untargetable, as he enters the body of the target, after 3 seconds or when this ability is reactivated, kayn appears coming out of the enemy in a small dash going in the direction of his mouse, dealing damage scaling off of his attack damage. Upon switching to darkin form, this ability will deal a percentage of targets max health as damage, scaling off of your attack damage, and also heal for a percentage of the target's max health, scaling off of your attack damage.

Starting area for jungle clear is basically always on raptors camp. If your team is starting on blue side(bottom left), i like to start on the enemies raptor camp, as it isn't commonly expected, and there isnt a bot lane to worry about roaming on you from that position. After taking their raptors, take their red buff as well, and then make your way back to your blue buff, then either start looking for ganks, or just clear the rest of your camps on your way back to your red side jungle.

Starting on the red side (top right) i basically start at the same position, on your teams raptor camp though. Pathing is similar, i like to go from raptors-red and from there either look for a gank toplane, or an invade around enemy blue. Invading around this time you can expect their jungler to be getting to their blue, making them an easy target if they are mid combat with it. If ganking and invading are both not an option, then head straight over to your wolves and get clearing that half of your jungle, then start looking botside for ganks/invades.