Full build focused around [R] last-hit damage and cooldown reduction.
Burst damage and target access
Damaging a lot health champion inflicts adaptive damage and harvests a soul form the victim.
Recover health when you damage an enemy champion.
Collect eyeballs for champion and ward takedowns. Gain permanent AD or AP, Adaptive for each eyeball plus bonus upon collection completion.
Unique takedowns grant permanent cooldown on your Ultimate.
Improved attacks and sustained damage
Takedowns restore 20% of your maximum mana and refund 10% of your ultimate's cooldown.
Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.
+6 AD or 10 AP
Dark Harvest's execution damage play pattern synergizes well with Karthus' [R] when you're looking to pick up late-hit kills.
Taste of Blood makes your lane phase sustain a lot more reliable.
Eyeball Collection's kill/assist stacking is easy to build up with a few early well-timed [R]s.
Ultimate Hunter is the perfect rune choice here for Karthus as the stacks are easy to pick up across the map with [R] as well as keeping your main ability on as short of a cooldown as possible.
Like Ultimate Hunter, this build is entirely focus around your [R] execute damage and keeping on as low of a cooldown as possible.
Coup De Grace further empowers your ability to lat hit champions with [R], especially when combined with Dark Harvest.