Hello and Welcome to my guide,Im from Germany and still learning the Game, still i would like to use this as a way of improving by thinking and talking more indepth about kayn in this Guide. Yet it probably wont be too helpful since i didnt include any Jungle clear nor Giving advices about ur Powerspikes still i hope it help, I'd love some comments ;)
Please forgive me for my (probably insanely) bad english.


Kayn in my opinions isnt very hard Mechanicly rather u have to know what to do since u have insane Adaptive abilities with his passive. This is why I will focus on ur Build and ur adaption to the opponents.

Smite Must have in Jgl
Ignite Helps ur Early a lot if u need a little Dmg or are up against very sustaining champs like Vladimir or Xin
Teleport Can be used if u want to babysit ur entire team or u know that u will splitpush
Exhaust Strong vs very good fighter and if ur team doesnt have Exhaust yet
Ghost giving u stickyness and chasing potential
Flash not really needed since ur mobility is extraordinary but u can use it to prevent dying from Mistakes.

Generally i go ignite because it also triggers Ur R (but doesnt give stacks for ur passive

My Runes atm
Quint of Attack Damage
Mark of Attack Damage
Seal of Armor
Glyph of Magic Resist

Because of your Great Scalings with ad and higher burst Generally these runes arent really what i thought about as much yet since u dont need to last hit ind jgl and u are doing quite some AA's ADC Runes could be viable 2.. espacially when preferring Raast/Darkin.

Reaping Slash
This is your main Spell, giving u Mobility as well as Damage. Remember it having 2 instances also Proccing The Blac Cleaver 2 times.

Blade's Reach
The only CC in ur kit this will mostlikely decide the outcome of ur Gank. Normally i would Q W but in Shadow Form remember u dont CC urself anymore while casting, which is why u should W Q for full Burst

Shadow Step
Giving u Sustain and Mobility. in combat the duration doesnt increase but still give u the same amount of health (and Slow resistance in Shadow form)

Umbral Tresspass
after Dealing damage u have 3 seconds to R the champ u damaged (Ignite and Redsmit count with each tick) This is why dodging skills can be quite Hard. Smite has a fairly higher Range than ur R. So if u eed to dodge either Blue Smite-R or Ignite R. This does not trigger Duskblade since u count as in Vision. Also u are not invincible in ur R only untargetable!

Enchantment Warrior If enchantment then this. U only scale with ad and u need the Dmg on ur Spells to be effctive Because of Both his Empowered Forms
Trackes Knife Use this when u have ignite and need more vision
Stalker's Blade Generelly the best because u can instantly Smite R and have tickyness for Raast as well as burst for Assassin
Skirmisher's Sabre Take this if u have to do long trades and go Raast with ADC runes
Hunter's Talisman Best joice since it also gives u 150% Mana reg and making u available to easily Clear solo with raptors start.
Refillable Potion Obviously better Clear and Goldefficiency
Health Potion + Control Ward alternate start if u somehow need an Early ControlWard
Stealth Wardu normally will start this
Sweeping Lens Lens if u want to invade early without beeing spotted or generally having op early vs enemies
FIRST BACK
Control Ward Just remember to always have one with u
Doran's Blade if u somehow need this early strengths, normally dont buy since u arent forced to 1v1 (like Riven top or something)
Caulfield's Warhammer U want this since it wll bild into Warrior enchantment/Duskblade/DeathsDance and u scale best with AD and CDR
Long Sword Another Option if u have Gold left or not enough for greater Item Enchantment: Warrior Best Enchantment for you, but not always needed to buy the jgl item
Poacher's Dirku got the Stronger Early? or just invade a lot, go for Poachers because of Gold efficiency
Skirmisher's Sabre Strong for exteended fights and 1v1's
Stalker's Blade Normally Taken (Burst+Stickyness)
Tracker's Knife U will probably need some Vision and if u have ignite for ur are why dont go for some wards
[[Boots of Speed] Upgrade into whatever u need most

Core

Black Cleaver giving CDR,Armor Pen and AD, also ur Q stacks TBC 2 times if u hit the dash
Enchantment: Warrior
Death's Dance Normally not building it on Shadow assassin because u need th early Lethality. On Raast it is ur First item since u will probably need some Damag and the Heal is op with thi (followed by Spirits Visage)

Shadow Assassin

Youmuu's Ghostblade Movementspeed for Roaming and Chasing potential
Duskblade of Draktharr Extra Burst
Edge of Night Utility and lil of tankyness
Last Whisper Probably u want to deal some Damage to tanks.. build it later Guardian Angel
Executioner's Calling isnt even needed that often since u want to burst them down anyways
Statikk Shiv+ Infinity Edge I tried it ad it actually is quite fun.. still i wouldnt take these normally
Hexdrinkervs Bursty fed mid or Top ap
Giant Slayer vs Tanks .. but u dont want to take Shadow assassin vs Tanks anyways

Darkin

Death's Dance Giving u incredible Sustain and a good early to mid game carry potential
Frozen Mallet Build this if u need to peel
Blade of the Ruined King Missing on Dmg/having enough Tankyness and beeing vs Loads of tanks
Ravenous Hydra I wouldnt Recommend it.. feels a little too much
Titanic Hydra U already have enough Waveclear still it can be viable
Sterak's Gage vs Mixed/ AD burst
Dead Man's Plate normally taken vs AD
Spirit Visage Always take this vs AP
Thornmail For the grevious Wounds primarilys just stay long with bramble west
Gargoyle Stoneplate + Locket of the Iron Solari If u feel really useless or the gme is about ur Carries to survive