Katarina Runes explained
There's one set of runes for Katarina on patch 15.3 that is the clear favorite for LoL players and pros alike. The best Katarina rune paths on the position 
Mid are 
Precision as the primary path and 
Domination as the secondary path with a pick rate of 63%. Besides picking runes on Katarina you should learn how to play Katarina and if you are interested in pro matches take a look on how a pro builds Katarina. If you want to discuss 
Katarina then you should visit the champion subreddit.
Katarina runes from primary path
Precision improves attack and sustained damage on Katarina troughout the match. Most of high elo players – and our LoL pros in the the major regions – have agreed that Conqueror is the best keystone rune to run on Katarina. This keystone rune provides additional power in the later stages of a game. As soon as Conqueror is fully charged, Katarina deals additional damage and gains life steal. Other primary runes on Katarina are Triumph, Legend: Alacrity and Last Stand.
Triumph restores on Katarina a part of missing health and award bonus gold, if a champion is killed. Legend: Alacrity grants additional attack speed to Katarina. With Last Stand you deal more damage while you are at low health.
Katarina runes from secondary path
When it comes to your secondary path, most Katarina players prefer to go with Domination. Domination improves burst damage and assassination tools on Katarina. On the Domination path, the best runes for Katarina are 
Sudden Impact and 
Relentless Hunter.
Sudden Impact increases your damage after dashing or being invisible. This is due to how easy it is to activate on Katarina 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. Relentless Hunter increases your movement speed out of combat due to every unique champion takedown.
FAQ for Katarina Runes
Which is the best keystone rune on Katarina?
The best keystone rune on Katarina is Conqueror. This Katarina runes setup is then referred as Katarina Conqueror.
What runes does Katarina use?
Runes from the paths Precision and Domination are the most used runes on Katarina. For the exact pick rates of Katarina runes used, you have to study the rune distribution presented.