Noxian by birth, Shieda Kayn and others like him were conscripted as child soldiers, a cruel practice employed by only the most devious commanders in Boram Darkwill’s empire. Following the disastrous battle at the Placidium of Navori, the invasion was deliberately reformulated into a protracted war of attrition. Ionian compassion was a weakness to be exploited—their warriors would hesitate before striking down a supposed innocent. Thus, barely able to lift the blade he had been given, Kayn’s first day in battle was also expected to be his last.

Striking against the province of Bahrl, Noxian forces landed at the mouth of the Epool River. Kayn and the others were a reluctant vanguard, facing disorganized bands of locals defending their home from these returning invaders. While his young comrades were cut down or fled the battlefield, Kayn showed no fear. He dropped his heavy sword and snatched up a fallen sickle, turning to face the shocked Ionians just as the Noxian regulars swept in from the flank.

The carnage was staggering. Farmers, hunters—even a handful of vastaya—all were butchered without ceremony.

Two days later, after word had spread throughout the southern provinces, the Order of Shadow came upon the grisly scene. Their leader, Zed, knew this area had no tactical significance. This massacre was intended as a message. Noxus would show no mercy.

A flickering glint of steel caught his eye. A child of no more than ten lay in the mud, leveling his broken sickle at the master assassin, bloody knuckles straining white. The boy’s eyes harbored a pain that belied his age, yet still burned with all the fury of a hardened warrior. This tenacity was not something that could be taught. Zed saw in this child, this abandoned Noxian survivor, a weapon that could be turned against those who had sent him here to die. The assassin held out his hand and welcomed Kayn into the Order of Shadow.

Acolytes traditionally spent years training with a single weapon of their choosing, but Kayn mastered them all—to him, they were mere tools, and he was the weapon. Armor he viewed as a cumbersome burden, instead cloaking himself in shadows and slaying his enemies with quickness and stealth. These swift executions instilled fear in the hearts of those fortunate enough to be spared.

And as Kayn’s legend grew, so did his arrogance. He truly believed that one day his power would eclipse even that of Zed himself.

This hubris led Kayn to embrace his final test: to seek out a darkin weapon recently unearthed in Noxus, and prevent it from ever being used against the weary defenders of Ionia. He accepted without hesitation, never questioning why he had been chosen for this task. Indeed, where any other acolyte would have destroyed the living scythe known as Rhaast, Kayn took it for himself.

The corruption took hold the moment his fingers closed around the weapon, locking them both in a fateful struggle. Rhaast has long awaited the perfect host in order to rejoin its darkin brethren and lay waste to the world, but Kayn will not be easily dominated. He returns to Ionia in triumph, convinced that Zed will name him the new leader of the Order of Shadow.

Passive: the Darin Scythe
you get points by damaging enemies
red points for Melee champions and Blue Points for Ranged ones
after getting a decent amount of points you can transform into Darkin or Shadow Assassin
Darkin: Heal for a percentage of the damage dealt to champions
Shadow Assassin: for the first few seconds in combat you deal bonus damage
Q:Reaping Slash
Kayn Dashes forward to strike nearby enemies. both dash and strike deal 65/85/106/125/145 + 65 percent of bonus ad
Darkin: Deal a percentage of the targets maximum health instead
(this ability deals bonus damage to minions and monsters)
W:Blade's Reach
Kayn deals 90/135/180/225/270 + 130 percent of bonus ad physical damage and slows enemies hit by 90 percents decaying over the next 1.5 seconds
Shadow Assassin:Gain Extra range and the ability to move during Blade's Reach
Darkin:Gain a knock up
E:Shadow Step
Kayn gets 40 percent movement speed and the ability to move through terrain for 7/7.5/8/8.5/9 seconds. when he first enters the terrain he is healed for 100/115/130/145/160 + 40 percent bonus ad
maximum duration in combat with enemy champions: 1.5 seconds
immobilizing effects or spending more than 1.5 seconds out of terrain will end Shadow Step immediately.
Shadow Assassin: Cooldown is reduced to 8 seconds and you get more movement speed and slow immunity
R:Umbral Trespass
Kayn infests an enemy champion he has damaged recently and becomes untargetable for up to 2.5 seconds also you can reactivate the ability to end it early. when umbral trespass ends the target takes 150/250/350 + 175 percent bonus ad physical damage
Shadow Assassin:Gains extra range both when entering and exiting the victim and refreshes the passive on exit
Darkin: Deals percent maximum health damage and heals for a percentage of the target maximum health

When to pick Darkin:
Darkin is better into tanks
he is a fighter so he is better in teamfights
he heals for a percentage of the damage dealt so he can stay alive longer
When to pick Assassin:
he is a better into Squishy targets
he is an assassin so he is better for skirmishes
he deals bonus damage in fights so he can kill much faster

you can Q near big walls and cancel it's dash animation while doing it
as Red Kayn you can Auto Attack someone when they are in the air and then Q so you get more Conqueror stacks and do more damage of course
Kayn can start without leash on raptors. just attack one of them and wait for them to allign perfectly and then Q in and they will burn then kill all the little ones
if enemy has no ranged champions or no ranged abilities at the moment you can recall at the top of the Red Buff pit bcuz no Melee champion can hit you there

for more tips just watch Karasmai videos. he is a kayn god.