Karthus Runes explained
There's one set of runes for Karthus on patch
12.10
that is the clear favorite for LoL players and pros alike. The best Karthus Runes on the position 
Jungle are Domination as the primary path and
Precision as the secondary path with a pick rate of 41%. Besides runes you should take a look on Karthus Build and if you are interested in pro matches take a look at Karthus Probuilds. If you want to discuss
Karthus then you should visit the champion subreddit.
Primary runes path on Karthus
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,
Eyeball Collection and
Treasure Hunter.
Cheap Shot makes you deal more damage to enemies with impaired movement, which has great synergy with the setup of Karthus.
Eyeball Collection is next up. This grants the user additional damage with every killing they score.
Treasure Hunter is the final rune of the
Domination path. This rune gives Karthus additional gold for takedowns. The additional gold is nothing to scoff at and can mean the difference between winning or losing a battle. Karthus will gain an additional 70 gold for the first takedown. Karthus acquires Bounty Hunter Stacks, which Karthus may use to get more takedowns. This rune's power will grow tremendously, and Karthus will become wealthy like never been before. Each time a fresh stack of Bounty Hunter Stacks is consumed, Karthus will get an additional twenty gold.
Secondary runes path on Karthus
When it comes to your secondary path, most
Karthus players prefer to go with
Precision.
Precision improves attack and sustained damage on Karthus troughout the match. On the
Precision path, the best runes for Karthus are
Presence of Mind and
Last Stand.
Presence of Mind helps Karthus with the mana problems by restoring missing mana, if a champion is killed. With
Last Stand you deal more damage while you are at low health.
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.