Katarina Runes explained
There's one set of runes for Katarina on patch
11.17
that is the clear favorite for LoL players and pros alike. The best Katarina Runes on the position 
Mid are Domination as the primary path and
Precision as the secondary path with a pick rate of 52%. Besides runes you should take a look on Katarina Build and if you are interested in pro matches take a look at Katarina Probuilds. If you want to discuss
Katarina then you should visit the champion subreddit.
Primary path on Katarina
Domination improves burst damage and assassination tools on Katarina. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Electrocute is the best keystone rune to run on
Katarina. This keystone rune provides burst damage, aiding them in nuking down the enemy carries. This is usually followed up by taking
Sudden Impact,
Eyeball Collection 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.
Eyeball Collection is next up. This grants the user additional damage with every killing they score.
Relentless Hunter increases your movement speed out of combat due to every unique champion takedown.
Secondary path on Katarina
When it comes to your secondary path, most
Katarina players prefer to go with
Precision.
Precision improves attack and sustained damage on Katarina troughout the match. On the
Precision path, the best runes for Katarina are
Triumph and
Coup de Grace.
Triumph restores on Katarina a part of missing health and award bonus gold, if a champion is killed. Katarina deals with
Coup de Grace more damage to targets with low health.