Kayn
Rhaast the Bruiser
Lane harassment and consistent damage output.
Domination
Burst damage and target access
ElectrocuteHitting a champion with 3 separate attacks or abilities in 3s deals bonus
adaptive damage .
Sudden ImpactDamaging basic attacks and abilities deal bonus true damage to enemy champions after using a dash, leap, blink, teleport, or when leaving stealth.
- Eyeball Collection
Collect eyeballs for champion
takedowns . Gain permanent AD or AP,adaptive for each eyeball plus bonus upon collection completion. 
Treasure HunterUnique
takedowns grant additional gold the first time they are collected.
Precision
Improved attacks and sustained damage
TriumphTakedowns restore 5% of your missing health and grant an additional 20 gold.
Coup de GraceDeal more damage to low health enemy champions.
Adaptive Force
+9
Adaptive Force Adaptive Force
+9
Adaptive Force 
Health Scaling
+10-180 Health (based on level)
Why Electrocute
Electrocute on Kayn grants you the burst damage needed to get going early with a kill or two to snowball the game in your favor.
Why Sudden Impact
Kayn is one of the biggest abusers of Sudden Impact as simply casting **[Q}** will grant you Lethality.
Why Eyeball Collection
Eyeball Collection is all about fighting from the get-go and getting takedowns to scale into the mid-game.
Why Treasure Hunter
Because Kayn likes to duel and spam his abilities, Ravenous Hunter provides a lot of extra healing in those fights. It also pairs nicely with Rhaast's passive.
WhyTriumph
Kayn's farly large AoE on [Q] and [W] allow you to hit multiple targets in teamfights, picking up assist/kills and grants you massive healing from Triumph in teamfights/skirmishes.
WhyCoup de Grace
Coup De Grace pairs well with Kayn natural assassin tendencies, and can greatly improve the damage of your [R] when looking to finish off a target.