Kayn Runes explained
There's one set of runes for Kayn on patch
11.8
that is the clear favorite for LoL players and pros alike. The best Kayn Runes on the position 
Jungle are Domination as the primary path and
Sorcery as the secondary path with a pick rate of 85%. Besides runes you should take a look on Kayn Build and if you are interested in pro matches take a look at Kayn Probuilds. If you want to discuss
Kayn then you should visit the champion subreddit.
Primary path on Kayn
Domination improves burst damage and assassination tools on Kayn. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Dark Harvest is the best keystone rune to run on
Kayn. This keystone rune gives you additional damage throughout the match. This is usually followed up by taking
Sudden Impact,
Eyeball Collection and
Ravenous Hunter.
Sudden Impact increases your damage after dashing or being invisible. This is due to how easy it is to activate on Kayn with the amount of ways he has to dash, jump or going invisible. The additional lethality helps to take down the priority targets that much easier.
Eyeball Collection is next up. This grants the user additional damage with every killing they score.
Ravenous Hunter is the final rune of the
Domination path. This rune gives some survivability during short fights and damage trades during the laning phase. The additional health is nothing to scoff at and can mean the difference between winning or losing a battle.
Secondary path on Kayn
When it comes to your secondary path, most
Kayn players prefer to go with
Sorcery.
Sorcery empoweres abilities of Kayn and increases the available champion resources. On the
Sorcery path, the best runes for Kayn are
Transcendence and
Gathering Storm.
Transcendence for some more Ability Haste.
Gathering Storm increases your late game power.