Karthus Runes explained
There's one set of runes for Karthus on patch 15.12 that is the clear favorite for LoL players and pros alike. The best Karthus rune paths on the position 
Jungle are 
Domination as the primary path and 
Sorcery as the secondary path with a pick rate of 63%. Besides picking runes on Karthus you should learn how to play Karthus and if you are interested in pro matches take a look on how a pro builds Karthus. If you want to discuss 
Karthus then you should visit the champion subreddit.
Karthus runes from primary path
Domination improves burst damage and assassination tools on Karthus. 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 Karthus. This keystone rune gives you additional damage throughout the match. Other primary runes on Karthus are 
Cheap Shot, 
Grisly Mementos and 
Ultimate Hunter.
Cheap Shot makes you deal more damage to enemies with impaired movement, which has great synergy with the setup of Karthus. Ultimate Hunter scales up Karthus ultimate haste on every unique champion takedown.
Karthus runes from secondary path
When it comes to your secondary path, most Karthus players prefer to go with Sorcery. Sorcery empoweres abilities of Karthus and increases the available champion resources. On the Sorcery path, the best runes for Karthus are 
Gathering Storm and 
Axiom Arcanist.
Gathering Storm increases your late game power. Axiom Arcanist grants you a shield, as soon as Karthus gets hurt by alot of magic damage. This rune will help you alot to survive against ability power champions with a lot of burst damage.
FAQ for Karthus Runes
Which is the best keystone rune on Karthus?
The best keystone rune on Karthus is Dark Harvest. This Karthus runes setup is then referred as Karthus Dark Harvest.
What runes does Karthus use?
Runes from the paths Domination and Sorcery are the most used runes on Karthus. For the exact pick rates of Karthus runes used, you have to study the rune distribution presented.